More GPU blur fixes.

- clamp the max. radius to 128, as the software path does
- use a more accurate radius-to-sigma conversion (0.6 instead of 0.6666)
- make SampleBlur derive from SampleView, not SkView, so benchmark mode ('f')
works
- implement a new BigBlur sample to test large radii

Review URL:  http://codereview.appspot.com/4726043



git-svn-id: http://skia.googlecode.com/svn/trunk@1871 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed