Per-tile glyph and icon atlases

[jfirebaugh]

Ports mapbox/mapbox-gl-native#9213; see that PR for extended explanation.

A breaking change to Map#addImage is included as a prerequisite; see the first commit.

Fixes #4876, fixes #141, fixes #4797.

benchmark	master 4d16dc2	atlases 05a7c5a
map-load	107 ms	76 ms
style-load	58 ms	50 ms
buffer	969 ms	970 ms
fps	60 fps	60 fps
frame-duration	6 ms, 0% > 16ms	6.1 ms, 0% > 16ms
query-point	0.58 ms	0.56 ms
query-box	35.91 ms	38.25 ms
geojson-setdata-small	3 ms	2 ms
geojson-setdata-large	111 ms	98 ms

Launch Checklist

• briefly describe the changes in this PR
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• Investigate style load performance regression
• Port Provide shrink as a public method shelf-pack-cpp#11 to shelf-pack JS: Provide shrink as a public method shelf-pack#90

[kkaefer]

Overall, this is tricky to review given its size and the many file renames. Could you please provide a short list of the fundamental changes/steps you did and whether there's anything in particular that you're concerned about or that could use a second pair of eyes?

[kkaefer]

Is there a particular reason for this change or just API cleanliness?

[jfirebaugh]

I made it internally at first, in order to simplify SpriteAtlas#addImage, and then realized it's just inherently a better design to separate required and optional parameters, and how we should have made the public API from the beginning. Though it's a breaking change, I think it's worth it now, with low usage. In the future it probably wouldn't be.

[jfirebaugh]

Sure.

• In broad outline, this PR follows the native PR fairly closely.

• In cases where new code had been written from scratch, I copied the C++ over, then adjusted the syntax for JS. That includes {image,glyph}_atlas.{cpp,js} and image_manager.js. It would include glyph_manager.js, but in JS there's the added ability to use TinySDF for local rendering, so JS deviates from native quite a bit.

• I also introduced a Texture class, like in native. (Preserve depth buffer between fill-extrusion layers + optimize render order #5101 adds RenderTexture; they should probably be combined/refactored at some point.)

• I introduced AlphaImage and RGBAImage, equivalents of native's UnassociatedImage and AlphaImage. There's no equivalent to PremultipliedImage because in a web context, non-premultiplied is the defacto standard due to the convention established by ImageData. The types work a little differently than in native: due to the constraints of Web Worker transfer, they can't be classes, so the "methods" are actually static functions that take "this" as their first parameter, and we do some casting to let flow treat plain old objects with the appropriate properties as "instances".

• In native, when loading the sprite sheet, we change it so that it's split up into individual images a while back. I ported that behavior.

• A significant cross-cutting change is to switch to image and glyph related types that follow native:

  Native	JS
  GlyphMetrics	GlyphMetrics
  Glyph	StyleGlyph
  style::Image	StyleImage
  UnassociatedImage	RGBAImage
  PremultipliedImage	no equivalent
  AlphaImage	AlphaImage
  GlyphPosition	GlyphPosition
  ImagePosition	ImagePosition

[kkaefer]

Overall this is a great change! Reviewed the code but didn't find any issues with it. It could benefit from some more comments that make the flow of information clearer.

[kkaefer]

Where does 3.0 come from?

[kkaefer]

I also introduced a Texture class, like in native. (#5101 adds RenderTexture; they should probably be combined/refactored at some point.)

Can you please ticket this so we can track it?