[sfnt] Fix color stop bounds check calculation at table end. Fixes https://bugs.chromium.org/p/skia/issues/detail?id=14021 * src/sfnt/ttcolr.c (VAR_IDX_BASE_SIZE): New macro. (tt_face_get_colorline_stops): Fix off-by-one bounds check calculation, take `VarColorStop` into account, and hopefully make it easier to read.