Add methods to access the top device's render target proxy

While the SDC-accessing versions have to stick around due to their
usage in unit tests, they will return null when exercising the NGA.
The render-target-proxy versions will work in both the OGA and NGA
and should, thus, be prefered.

Bug: skia:11837
Change-Id: I36213472e46e9da3ae6561f80d413090ceb6ad3e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/415058
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
10 files changed