Direct3D: support generation of BGRA mipmaps

Change-Id: I0d924a2bce1f1a90b2603c593b786bb1610e3d04
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449597
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
6 files changed