Remember which atlases were removed, and only
check those when looking for icons or glyphs
to remove. For most frames, we don't have to
check at all since no atlases were removed.
Use the same texture atlases to back both
the glyph and icon caches, and unify their
sizes and management. Store big glyphs
in separate textures, so all atlases have
the same size. Tweak some of the eviction
parameters.
We share the caches across all GL contexts
on a display, unless the GSK_NO_SHARED_CACHES
env var is set.
