Fix deadlock caused by nested thread groups in RaycastOcclusionCull

[jadeharley2]

Should fix deadlock which is described in 86306#issuecomment-2604466301

godot/modules/raycast/raycast_occlusion_cull.cpp

Line 427 in b15b24b

// Lots of instances, use per-instance threading

and

godot/modules/raycast/raycast_occlusion_cull.cpp

Line 432 in b15b24b

// Few instances, use threading on the vertex transforms

state that there is should be only one threaded group call in both branches, possibly to avoid deadlocks like described above. So the fix is to add a variable to check if we are already in a threaded call.

[jadeharley2]

I've messed up with git again, sorry.