…uct. r=gw

Doesn't make for a stellar API but texture_cache.rs has a lot of code and I had a hard time wrapping my head around the scattered parts of picture-cache specific code.

In addition (and more importantly) texture arrays for each tile sizes can be allocated on demand and do not need to be created when initializing the texture cache. This avoids allocating and deallocating the mispredicted picture texture size when the initial window size is garbage.

Depends on D67367

Differential Revision: https://phabricator.services.mozilla.com/D67368

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/955e7790f963c0c99ab8ab6ba537491a24023939