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/io.c

@@ -25,7 +25,7 @@
 // Fonctions de lecture/ecriture file, gèrent les systèmes big-endian et
 // little-endian.
 
-#include <SDL_endian.h>
+#include <stdio.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <errno.h>
@@ -53,6 +53,7 @@
 #else
     #include <dirent.h>
 #endif
+#include <SDL_endian.h>
 
 #include "struct.h"
 #include "io.h"