use blitMask for left/right edges in blur-nine

Seems about same speed for legacy blitter, but much faster for raster-pipeline

Bug: skia:
Change-Id: I19be307c01a199e2477e045fb8c2cca7784564a5
Reviewed-on: https://skia-review.googlesource.com/20967
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed