Rename GrXferProcessor::DstProxyView to GrDstProxyView.

At present, only Xfer processors allow reading back from the destination
image since they are in charge of blending. However, we'd like to expose
the destination color to fragment processors and Runtime Effects in the
future. To make this possible, the DstProxyView will need to be
accessible outside of Xfer processors.

This CL migrates DstProxyView to be a top-level Ganesh class and fixes
up the references to it throughout Skia. It's interesting to note that
several call sites were already using typedefs to hide the class
nesting anyway.

Change-Id: I93a294aa097f9319a968503c4f2f7e4f388ff033
Bug: skia:12066
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414899
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
64 files changed