add dstAT param to xform steps

All the existing calls are ported over, and new tests cover
unpremul output. (Opaque output is an impossible request.)

Change-Id: I744d640763cf74c368d3b3aba4a262c8fd9f7a01
Reviewed-on: https://skia-review.googlesource.com/147100
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
12 files changed