move SkImage::ColorType into SkColorType

objective -- move clients over to SkImage

tasks
- use SkImageInfo instead of SkBitmap::Config
- add support for colortables to SkImage
- add drawImage to SkCanvas
- return SkImage from readPixels

This CL works towards the first task

R=robertphillips@google.com

Review URL: https://codereview.chromium.org/54363008

git-svn-id: http://skia.googlecode.com/svn/trunk@12077 2bbb7eff-a529-9590-31e7-b0007b416f81
28 files changed