refactor SkRasterPipelineBlitter

This refactors the factories so that the create-from-paint factory is a
front-patch to the create-from-shader-pipeline factory.  Feature-wise,
we make the pre-baked shader pipeline responsible for modulating by
paint alpha; the factory only adds when creating from the paint.

We can fold the alpha into the colors in drawVertices, which makes it
run a bit faster, dropping the need for a scale_1_float runtime stage.
This causes a few invisible diffs on the "vertices" GM, but everything
else draws the same.

Change-Id: I3eeacc9aafbce2023ab18991bbb68c35645e9387
Reviewed-on: https://skia-review.googlesource.com/17395
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

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