Clean up remaining users of SkPMFloat This switches over SkXfermodes_opts.h and SkColorMatrixFilter to use Sk4f, and converts the SkPMFloat benches to Sk4f benches. No pixels should change here, and no code beyond the Sk4f_ benches should change speed. The benches are faster than the old versions. BUG=skia:4117 Review URL: https://codereview.chromium.org/1324743002