…. r=Bert,sotaro

This patch refactors how external surfaces are stored in the
CompositeState structure. This is primarily to simplify integration
with native compositor mode, but also simplifies the Draw compositor
path.

Previously, the ResolvedExternalSurface struct contained information
that was used to rasterize the external surface (YUV planes etc) and
also the information to composite it (device rect, clip rect, z_id).

Now, ResolvedExternalSurface contains just the information required
to rasterize the external surface, while the compositing information
is handled by adding the external surface as a regular tile. This
makes it possible to unify how external surfaces are drawn, via the
common draw_tile_list method.

Differential Revision: https://phabricator.services.mozilla.com/D65269

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/4bbb79cbf9cc9efc7d599d1fe8d68bf620601fc9