Separable blur: subpixel blurring. We approximate intermediate values of blur for even kernel siz…

Separable blur:  subpixel blurring.  We approximate intermediate values of blur for even kernel sizes by using a kernel of size N offset to the left in the first pass, a kernel of size N offset to the right in the second pass, and a centered kernel of size N + 1 in the third pass.  This required adding support for asymmetrical radii to the box blur.

Since this can only be done in the 3-pass blur for separable blurs, we turn off the optimization that drops to low quality for blurs of < 3 pixels.

Committed: https://code.google.com/p/skia/source/detail?r=6464

[Stephen White, 2012-11-16 16:31:35]

PTAL

[reed1, 2012-11-16 16:51:44]

lgtm