Limit (bitmap) glyph cache size

Limit (bitmap) glyph cache size

Now that bitmap glyphs (for vector clip masks) are stored in the glyph
cache, make sure it cannot grow indefinitely easily. Similar to the
bitmap cache, track approximate size of the cached elements and reset
if the cache exceeds a certain limit.
Also, reduce default bitmap cache size to 30 MB as we have essentially
two bitmap caches now. That's still plenty in all use cases where
caching matters.