support non-4:3 displays

[pphaneuf]

No description provided.

[pphaneuf]

I have this big HDTV in my living room, and on my 24" desktop monitor,
Quadra is looking a bit pathetic. Not only is it very low resolution, but
also has "pillarboxes" on both sides, since it's only 4:3.

We should make Quadra more flexible in its choice of aspect ratios. This is
very tricky, because the background would have to be clipped or scaled
appropriately, among other things.

This could also enable making the Quadra window resizable, when in windowed
mode.

We could make it a bit more simplistic at first, by keeping it "480p", but
in all likeliness, this item might require some serious rewriting of a lot
of the interface code, unfortunately.

But one day, I'll play Quadra on my HDTV with my gamepad (issue #19), and
it'll be awesome. ;-)