add ImageShader, sharing code with its Bitmap cousin

This is done by having abstracted the BitmapShaderContext to take a BitmapProvider, instead of just a bitmap. This allows us to share all of that code between SkBitmap and SkImage, since both are valid providers.

It also means that we can simplify SkImage_Base to not need a virtual for onNewShader, since ALL images can uniformly be turned into a shader now.

BUG=skia:

Review URL: https://codereview.chromium.org/1342113002
9 files changed