composeshader stages

needed to add two helper stages for composeshader

load_rgba, store_rgba

These just read/write the r,g,b,a registers to context memory, making no promise as to how the
memory is formatted (e.g. interleaved -vs- planar).

Note that we have similar existing stages, but they did not seem to suit:

constant_color

This guy loads 4 floats from memory, and splats them into registers. I need to load 4 entire
registers.

load_f32, store_f32

These offset where they read/write based on the 'x' register, plus they guarantee that the memory
will be interleaved ala SkPM4f.

Bug: skia:
Change-Id: Iaa81f950660b837bdb34416ab3e342d56a92239b
Reviewed-on: https://skia-review.googlesource.com/16716
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed