start on SkJumper lowp mode

Just 3 stages implemented so far:

  load_8888
  swap_rb
  store_8888

That's enough to make the shortest non-trivial pipeline
that you see in the new unit test.

Change-Id: Iabf90866ab452f7183d8c8dec1405ece2db695dc
Reviewed-on: https://skia-review.googlesource.com/18458
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
7 files changed