Auto merge of #7206 - glennw:fix-debug-panic, r=pcwalton

Fix "'PaintWorker' panicked at 'index 0 and/or 4 in `*` do not lie on…

… character boundary" when printing display list.

This whole piece of code seems a bit fragile, but it fixes the immediate problem for now.

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components/gfx/display_list/mod.rs

@@ -314,7 +314,7 @@ impl StackingContext {
 
             if opts::get().dump_display_list_optimized {
                 println!("**** optimized display list. Tile bounds: {:?}", paint_context.page_rect);
-                display_list.print_items("*".to_owned());
+                display_list.print_items("####".to_owned());
             }
 
             // Sort positioned children according to z-index.