Returns the y co-ordinate of
the mouse cursor.
coord is the y position of the mouse.
This function uses the main
canvas' position to calculate its offset, so if the canvas is
smaller than the screen the mouse position (0,0) will actually
be the top left of the canvas, not the top left of the screen.
This also means it is posible
for this function to return numbers that are negative or larger
than the canvas height.