Prepare for before commiting changes needed for SDL2

By default SDL1.2 is still used, but by chaning the config.make
option, SDL2 is used instead as a build-time option.

What works, what doesn't?

Working fine:

Joystick input

Mouse input

Keyboard input

Sound

Fullscreen mode-switching - now mostly working

Issues:

Fullscreen *mostly* (there is often some
flickering sometimes, switching back and forth fixes it.
Okay, I think this is might be related to the problems
with the splash screen.  There's a problem with the GL bounds
I think.  I'm not an OpenGL expert by any means, so I'm not
sure what the problem is exactly.  A bigger issue is changing
fullscreen resolutions from the in game menu only properly
resizes going down, going up leaves the "window" at the
previous size but like the flickering, switching to windowed
and back fixes it.

Splash screen - code is there and no longer prevents startup.
It works sometimes.  I tried writing a test outside the game
and it worked fine.  Something is strange, under X the window
doesn't get painted or contains random video memory, once the
game starts rendering it's fine.  It always works under
wayland (with the Wayland SDL video backend), see below.

Windows support is untested.  I tried not to break it, or
even change it where possible.

Wayland - it works, even the splash screen!
There were some issues resizing under Weston, possibly fixed
now, lack of Server Side Decorations makes it tricky anyway!
Worked fine under Enlightenment/Wayland.

Android/iPhone, obscure devices ...  Probably needs OpenGL ES ...
maybe it just works like Wayland.

There is a lot of code that could be simplified, in particular
it's possible the SDL2 Windows support works...?

Testers/reviewers more than welcome!  Especially help with the
GL issues mentioned above.