Fix GPU blur cacheing bug. Sometimes, the texture cache serves us textures of

Fix GPU blur cacheing bug.  Sometimes, the texture cache serves us textures of
different sizes for srcTexture and dstTexture (this is fair; they're supposed
to be approximate).  Code was assuming otherwise while downsampling; fix is to
reapply the scale on each downsample.   (Yes, I could cache these reciprocals
if and when they prove to be a hot spot).

Also, use setIDiv(w,h) everywhere for conciseness.

Review URL:  http://codereview.appspot.com/4798041/

git-svn-id: http://skia.googlecode.com/svn/trunk@1907 2bbb7eff-a529-9590-31e7-b0007b416f81