Fix thread safety of FT_New_Face, FT_New_Memory_Face and FT_Done_Face
"In multi-threaded applications it is easiest to use one FT_Library object per thread. In case this is too cumbersome, a single FT_Library object across threads is possible also, as long as a mutex lock is used around FT_New_Face and FT_Done_Face."
It is not well possible to use
FT_Library object per thread because cairo can call
_ft_done_face_uncached from any thread so this patch adds proper mutex locking around
FT_Done_Face. Mutex is defined alongside
Unfortunately this patch depends on !798 because it touches similar regions.