- shadows are now stable (in stable mode) when an IBL rotation is
used.
- fix the shadow transform option which didn't work when an IBL rotation
was used
- also use the x-axis as a reference for the "up" direction when
computing the light space matrix so that we don't fall into the
degenerate case when the light points straight down, which is a
common case
FIXES=[299310624]
1b0db0fca2