Fixed rendering of shadows under bitmaps.
Added an option to SkBlurMask to produce higher quality blurs.
The HQ blur option is propagated through SkBlurDrawLooper so that it can be invoked by WebKit for drawing shadows.
Added a shadow rendering test to gm.
Bugs fixed: 146, 150
Related Chromium bugs: 11153, 42654
Code review: http://codereview.appspot.com/4174049
git-svn-id: http://skia.googlecode.com/svn/trunk@818 2bbb7eff-a529-9590-31e7-b0007b416f81
