Uses the value in the src alpha channel to choose how
to set the dst pixel.
This is a part of a multi-part change to detect and
display gpu overdraw on Android.
CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Ubuntu-GCC-x86_64-Debug-NoGPU-Trybot
BUG:32370375
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5113
Change-Id: I07040929d8a46bbadd499dccec75eebef0e11d11
Reviewed-on: https://skia-review.googlesource.com/5113
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
