function release_button(button) {
        if (guac_touchscreen.currentState[button]) {
            guac_touchscreen.currentState[button] = false;
            if (guac_touchscreen.onmouseup)
                guac_touchscreen.onmouseup(guac_touchscreen.currentState);
        }
    }
Releases the given mouse button, if it isn't already released. Valid
button values are "left", "middle", "right", "up", and "down".

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