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