Cycles: Add debug pass showing average number of ray bounces per pixel

Quite straightforward implementation, but still needs some work for the split
kernel. Includes both regular and split kernel implementation for that.

The pass is not exposed to the interface yet because it's currently not really
easy to have same pass listed in the menu multiple times.

intern/cycles/kernel/split/kernel_scene_intersect.h

@@ -124,6 +124,7 @@ ccl_device void kernel_scene_intersect(
 	if(state.flag & PATH_RAY_CAMERA) {
 		debug_data->num_bvh_traversal_steps += isect->num_traversal_steps;
 	}
+	debug_data->num_ray_bounces++;
 #endif
 
 	if(!hit) {