Direct3D: enable texture copies

Copying textures has been implemented but the caps flag wasn't set. Also
adds some comments about format requirements.

Change-Id: I8b9b3b9c88c72fad44a3a7191fc17054d5def938
Bug: skia:10446
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/403599
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed