SkYUVAPixmaps::toLegacy() init unused mapping for missing alpha channel

Avoids uninit memory down the line. This wouldn't have cause incorrect
rendering but potentially could have eliminated a batching opportunity.

Change-Id: I5494f13f4a56418f6686d956db4b44ce239f2533
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317205
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed