This moves the LineAtlas from a shared texture that contained SDF dash patterns to use individual textures.
Previously, the texture space was limited to a texture of 512 pixels height. Dash patterns were never removed (and are still never removed as of this patch), which means that this texture could fill up for styles that use a lot of different dash patterns. In particular, dash patterns for lines with a round line cap take up 15 pixels of texture height, limiting the amount of unique dash patterns to 34. While this was probably enough for rendering a single style, we quickly exhausted this number when reusing the Map object to render different styles.
Instead of a global shared texture, we're now creating individual textures for every dash pattern. These textures are still cached so that we don't need to re-upload the texture on every frame.
This is first part of work on porting mapbox/mapbox-gl-js#8694 - in follow up patch(es) it is required to remove CPU side copy using 2d canvas support on all supported platforms, similar to approach taken in gl.js https://github.com/mapbox/mapbox-gl-js/pull/8694/files#diff-34dbe5f7de34dc4b9a8745dcde9bdc37R48
Decoding on CPU removed.
Padding is still done in DEMData() but, instead od doing it wwhile decoding, it is using memcpy to pad original values.
Rebase to latest mapbox-gl-js master and re-generate shaders.
Partly fixes: #15503
