Texture pages are not properly reused or deallocated

[staktrace]

When running the gecko webgl mochitest suite, WR memory usage climbs into multiple gigabytes (see https://bugzilla.mozilla.org/show_bug.cgi?id=1341745). I traced this down to the fact that WR periodically allocates a new TexturePage but never goes back and reuses old texture pages or deallocates them if they are empty. I checked with @kvark and he agreed that this appears to be a bug. Quoting him from IRC:

I don't see TexturePage objects being de-allocated. Also, they are not re-used properly. As in -
allocation only checks for the last page, while other pages may have more free space if they got
some content removed from them
so I think 2 modifications need to be done: 1) removal of the last page in a vector if it becomes
empty, and 2) check for all the pages when allocating

[kvark]

Note the comment:

// TODO(gw): Actually free items from the texture cache!!

[glennw]

committed on 29 Oct 2015

I guess it's time to fix that!

[kvark]

I guess I shouldn't try to actually remove the TexturePage from the arena then? Since doing so would potentially allow for a test case to constantly create and destroy textures each frame, and the real reason for the leak is that 2015 missing feature.

[glennw]

It'd probably be worth adding some profile counters that track some of the texture cache stats (e.g. total bytes allocated, page count etc) so that we can visually see if something is leaking in the future.

[staktrace]

@kvark thanks, the fix seems to work!