add entry-point to SkMaskFilter to fast-path rectangles.
have blurmaskfilter override this new method.

#define SK_IGNORE_FAST_BLURRECT if you want to disable this (not sure if we'll need this)
Review URL: https://codereview.appspot.com/6815087

git-svn-id: http://skia.googlecode.com/svn/trunk@6363 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed