move clip-stencil function into GpuDevice

Will follow-up with a change that doesn't use SkRegions!

Note:

The previous code flow gathered a region for all devices (in the canvas method).
However, it only tried to draw into the top device. The new code just focuses on
the top device, which ought to give the same results.

Change-Id: Ic5ed47e7908d646700c09b10faa538415522c645
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261283
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
5 files changed