Add an arg to SkImage::makeSubset to take a direct context

This is part of a larger effort to force users to provide a context
when manipulating GPU images which may be shared, instead of having
images themselves retain powerful context pointers.

Chrome flag landed in Chrome CL 2292800

Bug: skia:10466
Change-Id: Ic530a2c5eb1f4399db899d243ea944760fdf2055
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300707
Commit-Queue: Adlai Holler <adlai@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
18 files changed