GPU-based Gaussian blur.

GPU-based Gaussian blur.

This is a first stab at implementing a GPU-based
Gaussian blur in Ganesh.  The convolution shader is implemented as a new
filtering mode.  There are several known issues:

- no support for blur types other than "normal"
- FBO truncation problem at high zoom values
- uses bilinear for upsampling instead of Mitchell

Review URL:  http://codereview.appspot.com/4645082/

git-svn-id: http://skia.googlecode.com/svn/trunk@1830 2bbb7eff-a529-9590-31e7-b0007b416f81