plumb y through to SkJumper

There'll still be a little more refactoring after this, but this is the
main thing we want to do.

This makes y available in a general-purpose register in pipeline stages,
just like x.  Stages that need y (seed_shader and dither) can just use
it rather than pulling it off a context pointer.  seed_shader loses its
context pointer, and dither's gets simpler.

Change-Id: Ic2d1e13b03fb45b73e308b38aafbb3a14c29cf7f
Reviewed-on: https://skia-review.googlesource.com/18383
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
7 files changed