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>