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 :) ).
Merge pull request #1277 from dorkster/fixes

Don't let allies with 0 speed follow the player between maps
Merge pull request #1278 from dorkster/fullscreen

Nicer fullscreen when using SDL 2
Merge pull request #1280 from dorkster/fixes

Hide step sfx warning and fix actionbar item count display
When spawning enemies, don't block tile before checking requires_status
Fix spawn powers not working when their initial target was blocked
Use cooldowns from power definitions for enemy power cooldowns

This obsoletes the related cooldown keys in enemy definitions.