Move dest-texture readback code out of XferProcessor.

The destination texture readback now occurs before any fragment
processors run. This makes `_dstColor` accessible to fragment shaders.

Change-Id: Ifa5bd76de46414a21d7ae15ccf40938b4336429f
Bug: skia:12066
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/415737
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed