Factor out common work in readSurfacePixels and writeSurfacePixels

Use more consistent structure between the two functions.

BUG=skia:5853

Change-Id: I82465bee30025574d0d6fad9e287b187d75fff8d
Reviewed-on: https://skia-review.googlesource.com/10042
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
2 files changed