centralize RP loads and stores

This gets all the main places we set up loads and stores in
SkRasterPipeline.  Some tests and benches still use the old
generic append() method, and I think that's fine, and same
with some of the image codec code.

Change-Id: Ia27f106887f1a8d95d76e2366c17ab30eec504c1
Reviewed-on: https://skia-review.googlesource.com/154868
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed