Add xp optimization for RGB coverage.

This is needed since clearColorStages is being changed to ignore color input. For RGB coverage,
we want to clear all the color stages (since we know the final output color), but we don't want
to ignore the color input since it is needed.

In future we will change this so the XP stores the color internally and thus can tell the GP to
simiply ignore color.

BUG=skia:

Review URL: https://codereview.chromium.org/791933006

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