This fixes a case where the backing surface of a picture cache tile
changes, but was still being considered a no-op frame, which skips
the composite as an optimization if nothing has changed.

In this case, the tile surface changes from a rasterized texture
to a solid color that doesn't require a picture cache texture.

The patch ensures that the composite surface descriptors that are
used to detect no-op frame compositions include the backing
surface for each of the tiles that make up this virtual surface.

Differential Revision: https://phabricator.services.mozilla.com/D70138

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/5b784c47235ccd535d694c1ec2caf5b341570ff1