Notice style transitions that don't change how StThemeNode paints

Notice style transitions that don't change how StThemeNode paints

Add st_theme_node_paint_equal() and use that to do two things:

 1) Avoid animating transitions where nothing changes.
 2) Copy cached painting state from the old theme node to the new
    theme node.

https://bugzilla.gnome.org/show_bug.cgi?id=627083