support 888x, 1010102, and 101010x in SkPixmap::erase()

... and a few more methods to make it possible to write the new test.

Bug: oss-fuzz:6606

Change-Id: Ie8dd221059579248405f165a93c324c8ba518fd4
Reviewed-on: https://skia-review.googlesource.com/112400
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed