Choose background color and whether to draw mskp bounds in viewer

Default the background color to transparent black, which is correct for
mskps captures from Android framework.

Bug: skia:11900
Change-Id: I97cab04993c0f85259e831f3d0c44f9b962365af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/403077
Reviewed-by: Nathaniel Nifong <nifong@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed