Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv (take 2)

This is in service of: https://skia-review.googlesource.com/c/11125/ (Add parallel proxyID to StencilOps & RenderTargetOpList) where I want a better choke point for texture creation to improve discard handling.

This is a reland of: https://skia-review.googlesource.com/c/11200/ (Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv)

Change-Id: Icd0a90d2beb483dc24ed87c3bace9c817019e148
Reviewed-on: https://skia-review.googlesource.com/11326
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
30 files changed