Extract a GrRenderTask base class from GrOpList

This is a more abstract representation of something that can
participate in the DAG by modifying a GrSurfaceProxy's pixels. This
will serve as the direct base class for future DAG participants that
aren't opLists, and help to eventually remove GrTextureOpList.

Bug: skia:
Change-Id: Ia903c0188e0f6a6602781b90ee8da87ba3cc13f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/231277
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
26 files changed