on destroy, destroy all items (when built with USE_OPENGL version, notify OpenGL cache about removed glyphs)
clear usage flags for all entries
removes entries not used after last call of checkpoint() or cleanup()
removes all entries (when built with USE_OPENGL version, notify OpenGL cache about removed glyphs)
try to find glyph for character in cache, returns null if not found
put character glyph to cache
Recently used glyphs are marked with glyph.lastUsage = 1
checkpoint() call clears usage marks
cleanup() removes all items not accessed since last checkpoint()
Glyph image cache