add gpu impl for shadermaskfilter

adds MulChildAlphaByInput
renames MulOutputByInputAlpha --> MulChildByInputAlpha

Bug: skia:7500
Change-Id: Ic0615d4d23a887fbee510901ed77a36f98a1b11d
Reviewed-on: https://skia-review.googlesource.com/102440
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
9 files changed