We need this to properly trace Multiple Master CFFs, which contain
two SIDs that are charstrings.
This commit makes FreeType also show the last SID, omitted
previously due to a bug.
* src/cff/cfftypes.h (CFF_FontRec): Add `string_pool_size' field.
* src/cff/cffload.c (cff_index_get_pointers): Add argument to return
the pool size.
Update all callers.
* src/cff/cffobjs.c (cff_face_init) [FT_DEBUG_LEVEL_TRACE]: Directly
access `cff->strings' to display the non-default strings.