prepare to combine SDFTSubRun with SDFTSubRunSlug

* use GrTextReferenceFrame instead of blob
* use TransformedMaskVertexFiller
* remove mask field - this was always A8

Change-Id: I645a68658c4f873301db51987739e9138f8d4822
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502780
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed