Blender Git Loki

Cycles: make megakernel execution work again

After addition of more kernels the logic didn't work well anymore, for example
shade_light was holding up all paths going to the intersect_closest state.

Rather than trying to get all paths to the intersect_closest state, make the
megakernel support paths in different states.

This seems to help viewport rendering by about 10% here. I also tweaked the
threshold to what gave me best performance, but hard to say in general what this
value should be.

I noticed get_num_active_paths() used for adding more work tiles is not correct
since it's double counting some paths. Not fixing it as part of this patch, only
adding a comment about it.