[Direct3D] Check for null transfer buffer in readPixels.

Also changed to use the ResourceProvider so we can recycle buffers.

Change-Id: Idb7be607255e90669613bd8b68bf242d052997bc
Bug: skia:14255
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/671059
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
1 file changed