SkColorCubeFilter_opts: start with a statically-initializable zero.

SkColorCubeFilter_opts: start with a statically-initializable zero.

SkPMFloat(0) and SkPMFloat(0,0,0,0) end up with the same value,
but the first goes through math to get there.  The second is a lot more
transparent to the compiler, and  should compile all the way down to
just `xorps xmmN,xmmN` or even be optimized away.

Didn't measure any additional benefit from hoisting the zero outside
the loop and writing `SkPMFloat color = zero;`.

Perf win is <2%.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/435af2f736c85c3274a0c6760a3523810750d237

[mtklein_C, 2015-08-27 13:25:00]

mtklein@chromium.org changed reviewers:
+ noel@chromium.org

[Noel Gordon, 2015-08-27 13:31:50]

https://codereview.chromium.org/1314763007/diff/1/src/opts/SkColorCubeFilter_...
File src/opts/SkColorCubeFilter_opts.h (right):

https://codereview.chromium.org/1314763007/diff/1/src/opts/SkColorCubeFilter_...
src/opts/SkColorCubeFilter_opts.h:54: SkPMFloat color(0,0,0,0);
My earlier test patches did it this way too. Agree it it better.

[Noel Gordon, 2015-08-27 13:33:00]

LGTM.

Hoisting the zero might be a small win. Bigger fish to fry.

[mtklein, 2015-08-27 13:39:25]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-08-27 13:39:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1314763007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1314763007/1

[commit-bot: I haz the power, 2015-08-27 13:39:30]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-08-27 19:39 UTC

[mtklein, 2015-08-27 13:45:35]

wtf, lgtm

[commit-bot: I haz the power, 2015-08-27 13:46:06]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/435af2f736c85c3274a0c6760a3523810750d237

[Noel Gordon, 2015-08-27 13:46:25]

On 2015/08/27 13:45:35, mtklein wrote:
> wtf, lgtm

:)