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

HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,qrealka/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,shahrzadmn/skia,tmpvar/skia.cc,HalCanary/skia-hc,rubenvb/skia,google/skia,HalCanary/skia-hc,shahrzadmn/skia,tmpvar/skia.cc,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,tmpvar/skia.cc,tmpvar/skia.cc,qrealka/skia-hc,aosp-mirror/platform_external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,shahrzadmn/skia,google/skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,qrealka/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,google/skia,HalCanary/skia-hc,shahrzadmn/skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,qrealka/skia-hc,shahrzadmn/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,qrealka/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,qrealka/skia-hc,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,qrealka/skia-hc,rubenvb/skia,shahrzadmn/skia,google/skia,qrealka/skia-hc,shahrzadmn/skia,HalCanary/skia-hc,shahrzadmn/skia,HalCanary/skia-hc,tmpvar/skia.cc,HalCanary/skia-hc,shahrzadmn/skia,google/skia