Add Swizzle FP, and remove swizzle logic from GrConfigConversionEffect

Trying to decompose read/writePixels into some reusable chunks.

BUG=skia:5853

Change-Id: If08b004b008e8ca7d464d9dc47068e487bae850a
Reviewed-on: https://skia-review.googlesource.com/10015
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed