function press_button(button) {
        if (!guac_touchscreen.currentState[button]) {
            guac_touchscreen.currentState[button] = true;
            if (guac_touchscreen.onmousedown)
                guac_touchscreen.onmousedown(guac_touchscreen.currentState);
        }
    }
Presses the given mouse button, if it isn't already pressed. Valid
button values are "left", "middle", "right", "up", and "down".

@private
@param {String} button The mouse button to press.