Blender Git Loki

Cycles: Reduce memory used by background light update

Simple change: just get rid of intermediate data a bit earlier, before
final pixels array is being allocated. This gives around 30% of memory
save during light update (this is about 60meg in the frank sheep file
i'm using here).

This isn't really visible by artists a lot, because actual spike happens
on BVH construction. But it doesn't mean we shouldn't be accurate with
memory usage in other areas.