Cycles: Allow up to 4 motion curve primitives per BVH node

This avoids intersection AABB of different curve primitives
which makes it less ray-to-primitive intersections.

This gives about 30% speedup of hair rendering in the barber
shop scenes here. There is still some work to be done on those
files to solve major speed issues on certain frames.

intern/cycles/bvh/bvh_params.h

@@ -84,7 +84,7 @@ public:
 		max_triangle_leaf_size = 8;
 		max_motion_triangle_leaf_size = 8;
 		max_curve_leaf_size = 1;
-		max_motion_curve_leaf_size = 1;
+		max_motion_curve_leaf_size = 4;
 
 		top_level = false;
 		use_qbvh = false;