Implement srcRect and dstRect functionality in SkBitmapSource. This is required for...

Implement srcRect and dstRect functionality in SkBitmapSource. This is required for the "preserveAspectRatio" options of SVG's feImage. Covered by new GM "bitmapsource".

Note: I initially implemented this as a fully-generic SkResizeImageFilter, but then I realized that the dstRect should always be transformed by the filter matrix, but that the srcRect should not (since it's specified relative to the dimensions of the original bitmap). Since this would be confusing for someone attempting to use this as a generic resizing filter, I decided to build the functionality into SkBitmapSource instead.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/106933002

git-svn-id: http://skia.googlecode.com/svn/trunk@12522 2bbb7eff-a529-9590-31e7-b0007b416f81