Functions that utilize glyph cache should lock and unlock cache by calling `blf_glyph_cache_acquire()` and `blf_glyph_cache_release()`. Function `blf_glyph_cache_acquire()` will create glyph cache, if it doesn't exist. Locking mutex is global and shared by all fonts. Reviewed By: brecht Differential Revision: https://developer.blender.org/D5701