Draft: ShapeIcon: cleanup cosmetics + some performance

• Fix reference symbol style
• Pass pixbuf to paint_icon() by const reference, to avoid copy/refcount
• Drop redundant const on 1st template argument of std::map - always is!
• Replace manual surface management with Gdk::Cairo::set_source_pixbuf()
• Exit early instead of indenting if (not) clipmask > 0.
• Remove unnecessary, implied setting of RefPtrs to nullptr
• Move setting the MODE from every render, to just the constructor!