Move shader register setup to SkRasterPipelineBlitter.

Move shader register setup to SkRasterPipelineBlitter.

We've been seeding the initial values of our registers to x+0.5,y+0.5,
1,0, 0,0,0,0 (useful values for shaders to start with) in all pipelines.
This CL changes that to do so only when blitting, and only when we have
a shader.

The nicest part of this change is that SkRasterPipeline itself no longer
needs to have a concept of y, or what x means.  It just marches x
through [x,x+n), and the blitter handles y and layers the meaning of
"dst x coordinate" onto x.

This ought to make SkSplicer a little easier to work with too.

dm --src gm --config f16 srgb 565 all draws the same.

Change-Id: I69d8c1cc14a06e5dfdd6a7493364f43a18f8dec5
Reviewed-on: https://skia-review.googlesource.com/7353
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>