Added GrSurfaceContext and GrTextureContext

Added GrSurfaceContext and GrTextureContext

This lets copy-to-texture to be treated like copy-to-rt.
To match current behavior, though, copies to texture are
still executed immediately (forcing a flush).

Once MDB is enabled, copies to texture will be deferred.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5093

Change-Id: Icc0ce5435507a5f0a237c22eedef879824952367
Reviewed-on: https://skia-review.googlesource.com/5093
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>