fix winning line so it draws on top of final move

I was having some issues with getting the winning line drawn
correctly. The place_image() function uses the `onload`
function, which doesn't wait for the image to be loaded before
continuing execution.  Apparently I don't need to use `onload`
at all though - it works fine if I just specify the path to the
image in the `init()` function.
refactor draw_winning_line() to not use switch statement

With my new get_x/y_from_position() functions, I can draw the
winning line just from the winning combination.  In the winning
_combinations array, the middle square is always the middle
element, so the winning line can be drawn from winning_combinations[0]
to winning_combinations[2].
add comments to explain offset of