implement SkRasterPipelineBlitter::blitAntiH2()

The default blitAntiH2() calls blitAntiH() with two length-1 runs, but
for this blitter creating a small mask is better.  We can stamp both
pixels out with a single pipeline invocation.

Change-Id: If356975e85310a4545e54f2231a142d6e537944d
Reviewed-on: https://skia-review.googlesource.com/18581
Reviewed-by: Mike Reed <reed@google.com>
1 file changed