Extract VertexFiller and add AtlasDrawDelegate as GPU abstraction

Still need to decouple Ganesh/Graphite from AtlasSubRuns and
VertexFiller

Change-Id: I7f305f162751729da2b152781619b8ef6484ed39
Bug: skia:14317
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/711396
Reviewed-by: Herb Derby <herb@google.com>
15 files changed