Improve font cache

Improve font cache

Always write the information for touched glyphs every frame, to try and
avoid accidently depending on those being stable across atlas updates.

When the font cache is out of space, perform an emergency repack,
recreating the atlas with only the glyphs touched in the current frame.

This should address both changing requirements on the font cache over
time from the application, as well as inefficient packing from
incrementally building the atlas.