Change cmake findSDL module.

This version also looks for submodules, and I think it's easier to deal with to
adapt to SDL2. Also updated cmakelists to new usage.
Add some stuff to .gitignore
Actually look for SDL2 in the module.

I copied the cmake module from a couple commits ago to the SDL2 version, and
modified to look for SDL2 libs.
Update SDLKey to new name SDL_Keycode.

Some sed magic: find . -type f -exec sed -i 's/SDLKey/SDL_Keycode/g' {} +
Hack around SDL_WarpMouse prototype change

It now wants an SDL_Window as a parameter. I'm just passing a null pointer for
now, with an assertion to mark it.
Substitute SDL_BUTTON_WHEEL* for new wheel events

I tried to route the mouse wheel events through the same channel as before, as
they were treated as button presses in sdl1.
Comment out some gfx code

For now I'll try to avoid the gfx library, although I might need it in the end
to keep some features.

Also, some more renaming.
Comment out window state related event handling

All the window handling has changed in SDL2. Comment out the old code and add
an assertion to catch the new ones.
Comment out more sdl_gfx code, and update SetColors usage

SDL gfx is more used than I first saw. At this point I wanted to add it as a
dependancy, but it turns out that the author has changed the api to work with
renderer instead of surfaces, so tough luck. I mailed him as I think the old
api should be kept, but I'm going on with that commented out anyway.

Also, updated SetColors to SetPaletteColors.
Remove non-existent SDL_keysym header

I'm not sure what was in it, but it compiles just fine without it.
Repeat the sed magic to replace SDLKey, after rebasing.
Fix more SDL_BUTTON_WHEEL*
SDL_WM_GrabInput does no long exist.
Fix some more color related stuff.
Update all the screen related stuff.

I tried to substitute all the 'screen' related code with the new SDL2 renderer
stuff. I keep having a 320x200 surface which is blitted into, and this ends up
in a streaming texture which is then rendered. I try to make use of the render
logic size, but of course I could not test anything of this.
Fix definition collision and more sdl_setcolors.
Comment out some zoom stuff that I don't think I need.
Comment out some non-compiling input code.

I'll have to come back and review it, I just consider non essential for now.
Update code to get available vide modes.
Reenable sdl_gfx code.

Why do I need sdl2_gfx? I don't, as I need the gfx operations on surfaces,
which is what v1 did. So use that instead. However, I had to do a "dirty" hack
to make this work: modify the gfx headers to include sdl2 headers instead of
sdl1.