This simplifies code, and gives us more flexibility around sizing the
shared texture caches (since we can effectively grow/shrink it by units
of 1MB rather than 16MB).

Differential Revision: https://phabricator.services.mozilla.com/D10852

A big part of this is handling coalescing, so that if we end up creating
several new regions in a given frame, we won't allocate/blit/free
textures unnecessarily.

Differential Revision: https://phabricator.services.mozilla.com/D10853

Aside from general cleanliness, this allows us to factor the selection
logic into a helper without a widely-scoped mutable borrow on the entire
TextureCache.

Differential Revision: https://phabricator.services.mozilla.com/D10854

This is a strict improvement over the status quo, though we'll still
eventually consume as much memory as before, since we don't try to
evict at all until we hit the max size. We'll fix that soon, but it's
worth landing this separately because:
* It's useful to separate out regressions in the growing logic from in
  any new complex eviction logic.
* This patch alone is a very large (~100MB) win on AWSY.

Differential Revision: https://phabricator.services.mozilla.com/D10855