Add an index to SkGlyphPos

In the mask case, the original position is needed for the fallback case.
Add an index from the original sequence to lookup the source position
when needed.

Change-Id: I16ca08cab6be685af45e4ccabf3d1f76efd7e61e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206709
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed