Move discard and asyncRescaleAndReadPixels* to SkBaseGpuDevice

The goal here is to delegate these calls to the SkBaseGpuDevice-derived
classes so SkSurface_Gpu doesn't need to know about SDCs.

Bug: skia:11837
Change-Id: I28bea7ad1cf5d9fc0ce690b2bd70ff79e022f8d2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414998
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed