fix Surface_to_bytefield()

Each line of the SDL surface use "pitch" bytes.
see https://pulkomandy.tk/projects/GrafX2/ticket/171

src/sdlscreen.c

@@ -373,17 +373,11 @@ byte * Surface_to_bytefield(SDL_Surface *source, byte * dest)
   byte *src;
   byte *dest_ptr;
   int y;
-  int remainder;
 
   // Support seulement des images 256 couleurs
   if (source->format->BytesPerPixel != 1)
     return NULL;
 
-  if (source->w & 3)
-    remainder=4-(source->w&3);
-  else
-    remainder=0;
-
   if (dest==NULL)
     dest=(byte *)malloc(source->w*source->h);
 
@@ -393,7 +387,7 @@ byte * Surface_to_bytefield(SDL_Surface *source, byte * dest)
   {
     memcpy(dest_ptr, src,source->w);
     dest_ptr += source->w;
-    src += source->w + remainder;
+    src += source->pitch;
   }
   return dest;