Remove cross-fading for patterns and dashes

[mourner]

Closes #10605. Removes cross-fading for patterns and dashes, instead making them change instantly when zooming.

This behavior adds a lot of internal complexity. It makes data-driven pattern and dash implementations very hard to reason about. It is hard to test with render tests. I think it also doesn't look great. Instant transitions seem less noticeable for both dasharrays and patterns.

This PR, aside from removing a ton of internal complexity, makes it much easier for the Native team to port data-driven line-dasharray/line-cap. It also makes data-driven pattern/dasharray more performant because it's twice less GPU memory cc @galinelle. Also -2kb gzipped bundle is nice.

update: another big benefit noted by @karimnaaji:

Removing this might have quite a beneficial impact on terrain render cache, as this would reduce the need to re-render an entire tile to the offscreen FBO and instead keep it cached for more frames. [...] as long as you used at least one draped layer with patterns this could start showing fairly noticeable performance difference

The behavior change is subtle enough that it will be hard to notice in most cases:

before	after

• Figure out why line-dasharray appears slightly differently
• Fix a few render tests
• Make sure there is no leftover cross-fading logic anywhere to remove

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improved performance of patterns and line dashes, also improving their appearance when zooming</changelog>

[mourner]

OK, after ironing out various quirks, this is mostly ready — the only remaining issue is 14 failing tests related to slightly different rendering of line dashes. Looks like this comes down to this code:

mapbox-gl-js/src/shaders/line.fragment.glsl

Line 56 in 20e8fd2

float sdfwidth = min(dash_from.z * u_scale.y, dash_to.z * u_scale.z);

So SDF gamma is not interpolated like other properties, but rather uses the smaller of two dashes of a cross-fade. I'm not sure whether it's meant to be like this — the updated rendering looks slightly less blurry in places, but also has a few aliasing artifacts, but not a very noticeable difference overall. Should we update the expected images of these tests, or is there a way to make rendering more similar? @ansis

[mourner]

Settled on a comrpomise sdfgamma value that's closest to what we had before, bringing down the number of failing render tests from 14 to 3, which I'm updating the last commit.

[karimnaaji]

Really glad to see that many simplifications and potential for performance improvements! We should run that against benchmap to get some numbers against this branch.

[mourner]

Benchmarks to review:

• rendering-2d
• rendering-3d
• loading

They're mostly inconclusive or with a very tiny improvement, likely because our styles / locations in the benchmarks don't involve many dashed/patterned layers.

[karimnaaji]

👍 LGTM