DeferredTextureImageData low-bit-depth/dithering support

Cause DeferredTextureImageData functionality to support low bit depth
(4444, 565) image formats (with dithering).

Bug: 720105
Change-Id: Ie3b5768ebc393d9b0a5322461c722bf37c80b791
Reviewed-on: https://skia-review.googlesource.com/18945
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Eric Karl <ericrk@chromium.org>
6 files changed