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/main.c

@@ -631,7 +631,7 @@ int Init_program(int argc,char * argv[])
     return(0);
   }
   
-  Joystick = SDL_JoystickOpen(0);
+  //Joystick = SDL_JoystickOpen(0);
   SDL_EnableKeyRepeat(250, 32);
   SDL_EnableUNICODE(SDL_ENABLE);
   SDL_WM_SetCaption("GrafX2","GrafX2");