Default to opaque bitmaps (fixes several redraw issues)

atari/bitmap.c

@@ -117,6 +117,7 @@ void * bitmap_create_ex( int w, int h, short bpp, int rowstride, unsigned int st
 			bitmap->bpp = bpp;
 			bitmap->resized = NULL;
 			bitmap->rowstride = rowstride;
+			bitmap->opaque = true;
         } else {
 			free(bitmap);
 			bitmap=NULL;