add --skvm and '!' to viewer to toggle using skvm blitters

Change-Id: I71b81e89a871d9bd96534f1fb8b88fe25045685c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278417
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed