[usbcd36]

That code assumes that both motors (or sets of motors) move the robot in one direction when they are going forwards. In order to make it work for conventional systems, you need to reverse one of the sides. Simplest way to do that is subtract the value you assign to the PWM outputs from 254:

  p1_x = 255 - p1_y;

  p1_y = 255 - pwm05;



  pwm13 = pwm14 = 254 - Limit_Mix(2000 + p1_y + p1_x - 127);  // reverse this side

  pwm15 = pwm16 = Limit_Mix(2000 + p1_y - p1_x + 127);