SkNf_round, use it in store_565 and store_tables.

This gives us a place to bottleneck this sort of conversion.  Every time I try to use the rounding float -> int instructions, they're just a little slower than working the 1/2 into the scale with FMA.  Weird.

Change-Id: I7718112b234b4b38ba6af8fef59a47642021839a
Reviewed-on: https://skia-review.googlesource.com/5483
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
1 file changed