[skottie] Fix Sphere effect lighting for transparency

Colors are premultiplied, so the specular component (which is additive)
must be modulated by alpha explicitly.

Change-Id: If5f7b456b0ce518b1806e9b1e754229c3a6d0981
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/539559
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
2 files changed