Function ggez::graphics::set_screen_coordinates

pub fn set_screen_coordinates(
    context: &mut Context, 
    left: f32, 
    right: f32, 
    top: f32, 
    bottom: f32
) -> GameResult<()>

Sets the bounds of the screen viewport.

The default coordinate system has (0,0) at the top-left corner with X increasing to the right and Y increasing down, with the viewport scaled such that one coordinate unit is one pixel on the screen. This function lets you change this coordinate system to be whatever you prefer.