reorganize GrAtlasTextOp creation

Break the GrAtlasTextOp ctor into a mask version, and an SDFT version.
Reorganize geometry creation to allow more of the GrAtlasSubRun API
to be private.

Change-Id: Ic29d4026a15386d135e0fabc4f270810d1638b2d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303262
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed