Implement Gaussian blurs for images. The caller creates an an

Implement Gaussian blurs for images.  The caller creates an an
SkBlurImageFilter, sets it on an SkPaint, passes that paint to saveLayer(),
draws the primitives which are to be blurred, then calls restore(), which
applies the blur.  The blurs have separate sizes in the horizontal and vertical
direction.  This feature is GPU-only for now.

NB:  Due to the clipping change, there are slight pixel differences on the
blurs_gpu and shadows_gpu tests, so those will require rebaselining on all
platforms, as will some of the WebKit layout tests (TBD).

Review URL:  http://codereview.appspot.com/5322068/

git-svn-id: http://skia.googlecode.com/svn/trunk@2643 2bbb7eff-a529-9590-31e7-b0007b416f81