…colors. r=kvark

With this patch, tiles that are covered only by the opaque backdrop
primitive are detected and noted as solid colors.

Solid color tiles save memory and performance, because:
 - No texture slice is allocated as a render target for them.
 - No need to rasterize this tile.
 - Drawing the tile is done with the faster rectangle shader.

This already saves performance and GPU memory on quite a few
real world sites (esp. when running at 4k). However, the main
benefit of this will be once we enable picture caching on
multiple content slices and the UI layer. When this occurs, it's
important to avoid allocating tile buffers for all the solid
rectangle tiles that the UI layer typically contains.

Differential Revision: https://phabricator.services.mozilla.com/D41342

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/96075609026a3816cb7a6dcf9cf3795afe7f82d6