theme: Reset button style state when done drawing

We use a single style context to draw titlebar buttons, updating
its state according to each button's prelight state as necessary.
This assumes that the original state is neither ACTIVE nor PRELIGHT,
which means we need to reset the state after drawing to avoid
propagating the state of the last-drawn button.

src/ui/theme.c

@@ -890,6 +890,7 @@ meta_frame_layout_draw_with_style (MetaFrameLayout         *layout,
       cairo_restore (cr);
       if (button_class)
         gtk_style_context_remove_class (style, button_class);
+      gtk_style_context_set_state (style, state);
     }
 }