Make test lower-level, make const_cast more visible.

Make test lower-level, make const_cast more visible.

I can only think there's something funky going on with the hidden
const_cast inside SkRasterPipeline.cpp, or with the Sk4h -> Sk4f
conversion.  So make the const_cast visible and write the test
directly in halfs instead of floats.

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Release-GN-Trybot

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3003

Change-Id: I3a7e19ae165fce44f177b4968a63ec04e25c4b93
Reviewed-on: https://skia-review.googlesource.com/3003
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>