isolate dependencies to SDL

Do not access directly the SDL_Surface Screen_pixels
use new functions Get_Screen_pixel_ptr() Set_Screen_pixel(),
Get_Screen_pixel()
And SetPalette()

Signed-off-by: Thomas Bernard <miniupnp@free.fr>

src/buttons.c

@@ -49,7 +49,6 @@
 #endif
 #include <ctype.h>
 #include <sys/stat.h>
-#include <SDL.h>
 
 #include "const.h"
 #include "struct.h"