Improve opening animation performance

[passsy]

Opening Wiredash has been a bit laggy, most noticeably on iOS and macOS due to the shader jank.

While experimenting with different clipping behaviors Clip.antiAliasWithSaveLayer vs. Clip.antiAlias vs. Clip.hardEdge as recommended in #225 I found several points of improvement.

• setState was called way too often
• delay corner animation
• theme merging wasn't cached

Together, I saw a significant reduction of jank.

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Results

Test setup:
iPhone SE, iOS 15.1
Flutter version 3.3.0-0.1.pre on channel beta
I always completely restarted the app, cleared the timeline and only measured close and open (two taps).

Before improvements (debug build)

After improvements (debug build)

Save or not to save Layer

I did not notice a significant performance improvement when switching from Clip.antiAliasWithSaveLayer to Clip.hardEdge

Clip.antiAliasWithSaveLayer profile build

Clip.hardEdge profile build

Instead, Clip.antiAlias and Clip.hardEdge produce very noticeable artifacts (see below 👇), thus not changing this.

Clip.hardEdge vs. Clip.antiAlias vs. Clip.antiAliasWithSaveLayer