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/keyboard.c

@@ -22,6 +22,7 @@
 */
 #include <string.h>
 #include <ctype.h>
+#include <SDL.h>
 #include "global.h"
 #include "keyboard.h"