Fix when using Fl_Scroll inside Fl_Overlay_Window

src/drivers/Cocoa/Fl_Cocoa_Window_Driver.cxx

@@ -252,7 +252,7 @@ int Fl_Cocoa_Window_Driver::scroll(int src_x, int src_y, int src_w, int src_h, i
 {
   CGImageRef img = CGImage_from_window_rect(src_x, src_y, src_w, src_h);
   if (img) {
-    float s = Fl_Graphics_Driver::default_driver().scale();
+    float s = Fl_Surface_Device::surface()->driver()->scale();
     ((Fl_Quartz_Graphics_Driver*)fl_graphics_driver)->draw_CGImage(img,
                                       dest_x, dest_y, lround(s*src_w), lround(s*src_h), 0, 0, src_w, src_h);
     CFRelease(img);