…t occluded r=nical

Tiles that are occluded are generally never seen, or only seen
occasionally. To reduce the number of compositor surfaces:

 * Defer native surface allocation until after occlusion culling occurs.
 * If a tile has a native surface, then becomes occluded, drop the surface.

With this scheme, the number of unused native surfaces will always be 0
on a page that doesn't have scrolling. For a page that has a scrollable
region, there will be a small number of unused tiles retained. The unused
tiles are those that are (a) not occluded (b) not currently visible (c) are
in the display port. We retain these for a small amount of time in case
they get scrolled back on screen.

This makes the allocation patterns for native surfaces match the way
that picture cache surfaces are allocated for simple compositing mode.

Differential Revision: https://phabricator.services.mozilla.com/D51973

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/70442369cd48e37b05d2982f4109622cf8209bc6