4-at-a-time SkPMColor -> SkPMFloat API.

Please see if this looks usable.  It may even give a perf boost if you use it, even without custom implementations for each instruction set.

I've been trying this morning to beat this naive loop implementation, but so far no luck with either _SSE2.h or _SSSE3.h.  It's possible this is an artifact of the microbenchmark, because we're not doing anything between the conversions.  I'd like to see how this fits into real code, what assembly's generated, what the hot spots are, etc.

I've updated the tests to test these new APIs, and splintered off a pair of new benchmarks that use the new APIs.  This required some minor rejiggering in the benches.

BUG=skia:

Review URL: https://codereview.chromium.org/978213003
3 files changed