We can't just assume that the pointer we'se using as a cache key will
stay unique forever. The texture might be freed, and a later allocated
texture might have the same addres now, causing the cache to return
incorrect results.
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.
