Add a deferred copy surface (take 2)

Add a deferred copy surface (take 2)

This CL forces all GrSurface copies to go through a GrSurfaceContext (rather than GrContext).

There is a bit of goofiness going on here until read/writePixels is also consolidated in GrSurfaceContext and a proxy-backed SkImage/SkSurface is added.

This is a reland of https://skia-review.googlesource.com/c/5773/ (Add a deferred copy surface)

Change-Id: Ide560f569aede5e622420dc2f30eef76357d69f4
Reviewed-on: https://skia-review.googlesource.com/5939
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>