window-actor-x11: Check array bounds before accessing array

scan_visible_region() scans through each value of a uint8_t array and checks
whether that value is 255. Right now it always checks one value too much
though, resulting in a buffer overflow. Fix that by checking the array
bounds before actually accessing the array.

Found by running gnome-shell with address sanitizer and starting
GIMP.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2856>

src/compositor/meta-window-actor-x11.c

@@ -723,7 +723,7 @@ scan_visible_region (guchar         *mask_data,
           for (x = rect.x; x < (rect.x + rect.width); x++)
             {
               int x2 = x;
-              while (mask_data[y * stride + x2] == 255 && x2 < (rect.x + rect.width))
+              while (x2 < (rect.x + rect.width) && mask_data[y * stride + x2] == 255)
                 x2++;
 
               if (x2 > x)