skrpb: evaluate color filters for constant shaders once.

The simplest thing to do here is just run shader+color filter pipeline at
construction time to create a new constant color shader (replacing the paint
color).

This reduces a pipeline like:
  - constant_color  (paint color)
  - matrix_4x5
  - clamp_a
  - load_d_foo, xfermode, lerp, store_foo
to
  - constant_color  (paint color -> matrix_4x5 -> clamp_a)
  - load_d_foo, xfermode, lerp, store_foo

To implement this all, we add a new store_f32 stage that writes SkPM4f, and
finally get around to implementing Sk8f::Store4() (store while reinterlacing).
Sk4f::Store4() already exists for both SSE and NEON.

Next step: reduce simple constant_color -> store pipelines (src mode, full
coverage) into non-pipeline memsets.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2480823002

Review-Url: https://codereview.chromium.org/2480823002

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