Add a method to set the number of physics solver iterations in 3D

This is only for GodotPhysics, and adds a 3D counterpart to the 2D
method that was recently added.

servers/physics/physics_server_sw.cpp

@@ -1274,6 +1274,10 @@ void PhysicsServerSW::set_active(bool p_active) {
 	active = p_active;
 };
 
+void PhysicsServerSW::set_collision_iterations(int p_iterations) {
+	iterations = p_iterations;
+};
+
 void PhysicsServerSW::init() {
 	last_step = 0.001;
 	iterations = 8; // 8?