Simplify SkGpuBlurUtils.

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
Committed: https://skia.googlesource.com/skia/+/48343eea6f40a523306069e917ed9a1d6c39434c

[Stephen White, 2015-11-02 17:51:34]

Description was changed from

==========
Simplify SkGpuBlurUtils.

We compute a lot of { srcRect, dstRect } pairs in this code, but dstRect
always has a zero origin. So pass srcRect to convolve_gaussian(), and
let it compute the dstRect and an offset. This saves us having to
3-patch (soon to be 9-patch) both rects.

BUG=skia:4502
==========

to

==========
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
==========

[Stephen White, 2015-11-02 17:51:48]

senorblanco@chromium.org changed reviewers:
+ bsalomon@google.com

[Stephen White, 2015-11-02 17:52:09]

Brian: PTAL. Thanks!

[Stephen White, 2015-11-02 17:53:50]

The CQ bit was checked by senorblanco@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-02 17:53:59]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1430593006/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1430593006/40001

[commit-bot: I haz the power, 2015-11-02 18:04:10]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-02 18:04:10]

Dry run: This issue passed the CQ dry run.

[bsalomon, 2015-11-02 19:10:01]

lgtm

[Stephen White, 2015-11-03 13:06:56]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2015-11-03 13:07:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1430593006/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1430593006/40001

[commit-bot: I haz the power, 2015-11-03 13:07:49]

Committed patchset #3 (id:40001) as
https://skia.googlesource.com/skia/+/48343eea6f40a523306069e917ed9a1d6c39434c