Issue 02

I probably wouldn't use the window's height/width.
For example, we might add a footer or something later, and using the window's size would cause the lines to draw over the footer. 

You could also make this code more elegant by using the fact that points are immutable:
`window.drawLine(Colour.WHITE, topLeft.addXY(...), topLeft.addXY(...));`
but that's up to you.