Nicer fullscreen when using SDL 2

We now use the native screen resolution at all times and use SDL to
scale to that resolution. The old method of changing video mode didn't
work well in some cases (I got a weird bug where X wanted to make
800x600 require panning on my monitor). This also makes ALT-Tabbing
better, since there's no flicker of resolution change (or worse, getting
the desktop stuck at a non-native resolution).

We can also run Flare at fullscreen resolutions *higher* than the user's
desktop resolution. Since I use a generic TV as a monitor through VGA,
SDL reports that I can use 1792x1344 as a resolution (the native
resolution is 1360x768). I can now use that resolution in fullscreen if
I want to (everything is really tiny :) ).