add alternate gather() impl.

Didn't really find anything interesting.  The two perform essentially
identically, with the portable version using a lot more code size to
do so.

Change-Id: Iaa4e05d15dd0773a47f20df89a11730b3f27199e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244867
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed