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:

Review URL: https://codereview.chromium.org/1314763007