Polish duotone rendering code

[ajlende]

What?

Part 4 in a set of duotone php refactoring to fix small issues in duotone rendering.

• Part 1: Port colord to PHP
• Part 2: Deprecate remaining global duotone functions
• Part 3: Group all duotone outputs
• Part 4: Polish duotone rendering code (this PR)
• Part 5: Refactor duotone class to allow for multiple instances

This part polishes things up while keeping the other 3 PRs a bit more readable.

Why?

Gotta shave those yaks.

How?

• Corrects a number of misleading comments.
• Adds better examples.
• Renames some private methods.
• Reorder methods to group related things and help the reading flow.

Testing Instructions

1. Ensure that duotone filters in all of the example content render the same in the post editor, site editor, and frontend.
2. Test that the custom filters render in classic themes. You can use the same example content which includes some.

Example content for TT3 block-out, canary, and pilgrimage variations

<!-- wp:heading -->
<h2 class="wp-block-heading">Image core preset</h2>
<!-- /wp:heading -->

<!-- wp:image {"style":{"color":{"duotone":"var:preset|duotone|midnight"}}} -->
<figure class="wp-block-image"><img src="https://loremflickr.com/640/360" alt="placeholder image" class=""/><figcaption class="wp-element-caption">Preset slug: midnight</figcaption></figure>
<!-- /wp:image -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Image theme preset</h2>
<!-- /wp:heading -->

<!-- wp:image {"style":{"color":{"duotone":"var:preset|duotone|default-filter"}}} -->
<figure class="wp-block-image"><img src="https://loremflickr.com/640/360" alt="placeholder image" class=""/><figcaption class="wp-element-caption">Preset slug: default-filter</figcaption></figure>
<!-- /wp:image -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Image custom filter</h2>
<!-- /wp:heading -->

<!-- wp:image {"style":{"color":{"duotone":["rgb(92, 51, 10)","#fcf2e8"]}}} -->
<figure class="wp-block-image"><img src="https://loremflickr.com/640/360" alt="placeholder image" class=""/><figcaption class="wp-element-caption">Custom sepia filter</figcaption></figure>
<!-- /wp:image -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Image unset</h2>
<!-- /wp:heading -->

<!-- wp:image {"style":{"color":{"duotone":"unset"}}} -->
<figure class="wp-block-image"><img src="https://loremflickr.com/640/360" alt="placeholder image" class=""/><figcaption class="wp-element-caption">This should never have a filter applied</figcaption></figure>
<!-- /wp:image -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Image theme.json styles (plain block)</h2>
<!-- /wp:heading -->

<!-- wp:image {"style":{"color":{}}} -->
<figure class="wp-block-image"><img src="https://loremflickr.com/640/360" alt="placeholder image" class=""/><figcaption class="wp-element-caption"><code>settings.styles.blocks['core/image'].filter.duotone</code></figcaption></figure>
<!-- /wp:image -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Cover core preset</h2>
<!-- /wp:heading -->

<!-- wp:cover {"url":"https://loremflickr.com/640/360","dimRatio":0,"style":{"color":{"duotone":"var:preset|duotone|midnight"}}} -->
<div class="wp-block-cover"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-0 has-background-dim"></span><img class="wp-block-cover__image-background" alt="placeholder image" src="https://loremflickr.com/640/360" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size">Preset slug: midnight</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Cover theme preset</h2>
<!-- /wp:heading -->

<!-- wp:cover {"url":"https://loremflickr.com/640/360","dimRatio":0,"style":{"color":{"duotone":"var:preset|duotone|default-filter"}}} -->
<div class="wp-block-cover"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-0 has-background-dim"></span><img class="wp-block-cover__image-background" alt="placeholder image" src="https://loremflickr.com/640/360" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size">Preset slug: default-filter</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Cover custom filter</h2>
<!-- /wp:heading -->

<!-- wp:cover {"url":"https://loremflickr.com/640/360","dimRatio":0,"style":{"color":{"duotone":["rgb(92, 51, 10)","#fcf2e8"]}}} -->
<div class="wp-block-cover"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-0 has-background-dim"></span><img class="wp-block-cover__image-background" alt="placeholder image" src="https://loremflickr.com/640/360" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size">Custom sepia filter</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Cover unset</h2>
<!-- /wp:heading -->

<!-- wp:cover {"url":"https://loremflickr.com/640/360","dimRatio":0,"style":{"color":{"duotone":"unset"}}} -->
<div class="wp-block-cover"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-0 has-background-dim"></span><img class="wp-block-cover__image-background" alt="placeholder image" src="https://loremflickr.com/640/360" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size">This should never have a filter applied.</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

<!-- wp:heading -->
<h2 class="wp-block-heading">Cover theme.json styles (plain block)</h2>
<!-- /wp:heading -->

<!-- wp:cover {"url":"https://loremflickr.com/640/360","dimRatio":0,"style":{"color":{}}} -->
<div class="wp-block-cover"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-0 has-background-dim"></span><img class="wp-block-cover__image-background" alt="placeholder image" src="https://loremflickr.com/640/360" data-object-fit="cover"/><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…"} -->
<p class="has-text-align-center">This depends on <code>settings.styles.blocks['core/cover'].filter.duotone being set</code> to the CSS custom property for a preset.</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

[github-actions]

Flaky tests detected in 498a2b0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4756540374
📝 Reported issues:

• [Flaky Test] should open block tab by default if a block is selected #36671 in specs/site-editor/settings-sidebar.test.js

[scruffian]

LGTM