Revert float xfermodes back to Sk4f (from Sk8f).

Generally this was a performance win, even on devices without AVX due
to unrolling, but on ARM+NEON it looks like that unrolling hurt a bit.

    while (...) { blend a pixel }
~~~>
    while (...) { blend two pixels }
    if (n % 2) { blend last pixel }

BUG=chromium:555278
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1465483002
1 file changed