Added asNewCustomStage to SkShader and implemented it for all the gradient shaders.

Not actually hooked up yet, but it should be ready to replace asABitmap for a future CL.
Review URL: https://codereview.appspot.com/6351113

git-svn-id: http://skia.googlecode.com/svn/trunk@4702 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed