[canvaskit] Handle Premul textures better

See https://github.com/flutter/flutter/issues/106433

Change-Id: Ib11fcd208d2c2c275a5119edbd2678a058ef4b22
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/553523
Reviewed-by: Greg Daniel <egdaniel@google.com>
8 files changed