extend opacity tests in SkVM blits

- Have SkVMBlitter also check the color type for opacity,
  and commit to this opacity as a guarantee rather than optional
  optimization.
- Have SkImageShader do the same when sampling.

This will make it easier to work with opaque formats like 888x, 101010x,
and should mean sampling from opaque images is a bit cheaper, skipping
any work to unpack and convert alpha.

Change-Id: I0d20418f8335bd07c8b1a80b042636424854df18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270607
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed