Return glyph bounding rect from prepareForMaskDrawing

This is the first step of hoisting calculation up the stack so
that entire glyphs don't need to be passed to SubRun creation.

Eventually, only packed glyph IDs and top-left points will be
passed to SubRun creation.

Change-Id: I34c5cd67a420229feab020803700f28733da6c4a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/563739
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed