Prefer the NV_framebuffer_blit extension over ANGLE

The ANGLE/CHROMIUM glBlitFramebuffer requires full size blits, which
is really bad for DMSAA. The NV extension does allow blits of
sub-rectangles, so we prefer that one if it's available.

Bug: skia:12176
Bug: skia:12177
Bug: skia:12234
Bug: skia:12235
Change-Id: I463ac631fff64ad564f294cf2f26be7410e8c356
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/430576
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed