SkPixelInfo::CopyPixels() refactor part 0: only one memcpy() case

I envision this eventually looking like the following:
Fast Path 1: memcpy
Fast Path 2: Legacy RGBA/BGRA unpremul/premul
Fast Path 3: SkColorSpaceXform
Fallback: Raster pipeline

BUG=skia:6021

Change-Id: I3f0e36b08b051ab4ba3949f04474f20d0c20a963
Reviewed-on: https://skia-review.googlesource.com/7435
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
3 files changed