increase alignment of half-float pixels in F16Stages test

I think the Chorizo is now enforcing that our aligned loads are really
aligned.  If this sticks, I'll follow up with the rest of the tests
disabled on the bug.

Bug: skia:7497

Change-Id: Id392e20ead395474f716a2c32d2643c801e03a2e
Reviewed-on: https://skia-review.googlesource.com/97202
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed