Simplify SkGpuBlurUtils.

We compute a lot of { srcRect, dstRect } pairs in this
code, but they're always the same width and height, and
dstRect often has a zero origin. So pass only one or
the other and an offset, where required. Among other
things, this simplifies the code in convolve_gaussian(),
since we only have to 3-patch the dstRect.

BUG=skia:4502

Review URL: https://codereview.chromium.org/1430593006
1 file changed