This fixes blurry icons for the menu button. Concerning the default size of the wnck icons, it is consistent with what was done for the panel (tasklist, windowmenu) in 4.18.1 (see e.g. xfce/xfce4-panel@3fe42737).
For the min/max/close button icons, I think it's better to leave them as is as long as they are loaded at a fixed size. They are blurry at 2x scale, but at least they are scaled. If you go through a
cairo_surface_t without doubling the pixbuf size, they are going to be sharp, but tiny.