set SkJumper_kMaxStride to 16

It's properly 16 today because of HSW/lowp stages handling 16 pixels at
a time, but it hasn't yet had an effect on lowp so we didn't notice.

As we add lowp shader stages this will start to matter,
so might as well bump it up to 16 now.

(One day _skx lowp stages could bump this up to 32.)

Change-Id: Idd8185c08e12dc657389a35bf659662c9670f98a
Reviewed-on: https://skia-review.googlesource.com/60565
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed