Start moving towards SDL2

Note that nothing will compile yet.
SDL2 compiles now (still doesn't work)
Fix to work with SDL2

Also add a more user-friendly error message should things die on startup
Finish ripping out open OpenGL/scaling code

All that junk is now handled by SDL2 itself and is no longer our problem
Oh, hey, SDL has their own API for this
Correct 320x200 aspect ratio, fix mouse scaling

Also some further removal of no-longer available options
Fix the build under Mac OS X
Remove no-longer needed SDLMain
Merge branch 'master' into sdl2
Restore mouse wheel support

Also fix defaults to default to fullscreen (since that's now windowed
fullscreen) and change another startup error to use the new
show_startup_error function