Introduce mergeGlyphAndImage to SkStrike

This call maintains image invariants while merging in
an image. It replaces much dubious code.

Change-Id: Id57fa1169f8944b2d10063fde983cd9d8256b598
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223303
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed