fix shadows when there is no receivers in the frustum
fix shadows when there is no receivers in the frustum

this case could cause an assert in debug builds. There was a few bugs:

- when computing the intersection between a box and a frustum, a special
  case test had a typo which would cause extra work in some case. Also,
  we were not detecting correctly when there was no intersection at all.
  This could result in some intersection point being found.

- Finally, we reject the intersection volume when it has only 3 vertices
  because that's not a volume.

Fixes #5933
google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament