start on raster pipeline 2d mode

  - Add run_2d(x,y,w,h) and start_pipeline_2d().
  - Add and test a 2d-compatible store_8888_2d stage.

Change-Id: Ib9c225d1b8cb40471ae4333df1d06eec4d506f8a
Reviewed-on: https://skia-review.googlesource.com/24401
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
8 files changed