allow mixer to defer to a shader

- reverse sense of Blend mixer to be (dst, src, mode)
- change pipeline register convention to be (dst, src)

Bug: skia:
Change-Id: I1f1d990001ad941cb8d44e1488c5f14077e181f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201386
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
9 files changed