Remove GrRenderTargetOpList::discard & move functionality to GrRenderTargetContext

Change-Id: If84ffb7b01543a44179837d683c0a4c004e52f63
Reviewed-on: https://skia-review.googlesource.com/13335
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed