Small downsample_3_3 optimization

Just a little less math...

mipmap_build_2047x2047_0_gamma
Before: 15.8ms
After:  13.4ms

Hard to see much difference on the gamma correct
mips, which are more dominated by load time.

BUG=skia:

Change-Id: I978cbc85a7d75cfcca2d5cd3fbc75e93413782f2
Reviewed-on: https://skia-review.googlesource.com/9988
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed