anchor: love.touchpressed
  Callback function triggered when the touch screen is touched.
  love.touchpressed( id, x, y, dx, dy, pressure )
  id light userdata The identifier for the touch press.
  x number The x-axis position of the touch press inside the window, in pixels.
  y number The y-axis position of the touch press inside the window, in pixels.
  dx number The x-axis movement of the touch press inside the window, in pixels. This should always be zero.
  dy number The y-axis movement of the touch press inside the window, in pixels. This should always be zero.
  pressure number The amount of pressure being applied. Most touch screens aren't pressure sensitive, in which case the pressure will be 1.