Simplify shader paint alpha modulation.

Simplify shader paint alpha modulation.

We don't need to do any of this funky swap, load, srcin nonsense.  We've got a perfectly good scale_constant_float stage just perfect to be used instead.

While we're at it, we only need to modulate by paint alpha if the paint's not opaque.  x*1== x...

This puts the (x,y) inputs to shaders in (r,g) where they expect them.  It also frees (dr,dg,db,da) for use by the shader.  Might be handy for bilerp.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5100

Change-Id: Ief60c469ecac8300798b67cc68817cc1d127cf17
Reviewed-on: https://skia-review.googlesource.com/5100
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>