Replace raster pipeline nextafter() calls with SkNu nudging

Replace raster pipeline nextafter() calls with SkNu nudging

(courtesy of mtklein@)

  nanobench -m gradient_linear_clamp\$ --config f16 --ms 2000 -q

Before: 753.66
After:  658.69

R=mtklein@google.com,herb@google.com

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: Ie1442da340f6cfc9aef65bec1f114c0e5db89fcb
Reviewed-on: https://skia-review.googlesource.com/7351
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>