use the right visual when we create the pixmaps

gfx/src/gtk/nsRenderingContextGTK.cpp

@@ -517,7 +517,7 @@ NS_IMETHODIMP nsRenderingContextGTK::CreateDrawingSurface(nsRect *aBounds,
   g_return_val_if_fail (aBounds != NULL, NS_ERROR_FAILURE);
   g_return_val_if_fail ((aBounds->width > 0) && (aBounds->height > 0), NS_ERROR_FAILURE);
  
-  pixmap = ::gdk_pixmap_new(mRenderingSurface->drawable, aBounds->width, aBounds->height, -1);
+  pixmap = ::gdk_pixmap_new(mRenderingSurface->drawable, aBounds->width, aBounds->height, gdk_rgb_get_visual()->depth);
   nsDrawingSurfaceGTK * surface = new nsDrawingSurfaceGTK();
 
   surface->drawable = pixmap;