[GPOS] Fix mark-to-mark positioning when one of the marks is a ligature

This commit: a3313e54008167e415b72c780ca7b9cda958d07e broke MarkMarkPos
when one of the marks itself is a ligature.  That regressed 26 Tibetan
tests (up from zero!).  Fix that.  Tibetan back to zero.
8 files changed