Add closest_power_of_2 func and implement mix_rate/latency on OS X

modules/visual_script/visual_script_builtin_funcs.cpp

@@ -915,7 +915,7 @@ void VisualScriptBuiltinFunc::exec_func(BuiltinFunc p_func, const Variant **p_in
 
 			VALIDATE_ARG_NUM(0);
 			int64_t num = *p_inputs[0];
-			*r_return = nearest_power_of_2(num);
+			*r_return = next_power_of_2(num);
 		} break;
 		case VisualScriptBuiltinFunc::OBJ_WEAKREF: {