Only drop targets from the render target pool when the size of
the pool is larger than an arbitrary threshold (this is 32 MB
for now), _and_ the render target hasn't been used in the last
60 frames of rendering.

This reduces the number of allocation thrashing of textures in
the render target pool on most pages.

Differential Revision: https://phabricator.services.mozilla.com/D70782

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/c6ed30c910c03820a01baec4dd7e4884e8e37cac