Clamp lighting dot product in paint_alpha_normals_rt

Fixes inconsistent results in other color types/spaces, where the output
can be negative.

Change-Id: I8a346cf7bb02cb298c11d658948c117d74b6ddaa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/469359
Auto-Submit: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed