Make sure random unit test processors use an alpha type compatible with
texture data.

It'd probably be good to provide an opaque texture that has an alpha
channel and widen the range of color types that are present in the
future.

Change-Id: I5587a479f7e2f3991f8ae27a4cd44afc533324e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260696
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
22 files changed