Expose the GPU side drawable

Create the new class SkPictureBackedGlyphDrawable, and
expose it at top-level for testing.

Bug: chromium:1442140
Change-Id: Ib65b9d7f392fda4c801ec23dad311a40899f009f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/695936
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed