GLUT Mouse Callback

Mouse-button callback functions take the form:

    def func(button, state, x, y):
         ...

button is the button pressed or released - one of GLUT_LEFT_BUTTON, GLUT_MIDDLE_BUTTON, or GLUT_RIGHT_BUTTON

state is GLUT_DOWN if the button was pressed, or GLUT_UP if it was released

x, y is the mouse pointer location when the button was pressed or released