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
const int i10 = (colorToIndex[1][g] + colorToIndex[0][b] * dim) * dim;
const int i11 = (colorToIndex[1][g] + colorToIndex[1][b] * dim) * dim;
- SkPMFloat color(0);
+ SkPMFloat color(0,0,0,0);
for (int x = 0; x < 2; ++x) {
const int ix = colorToIndex[x][r];