st/texture-cache: Return a GIcon from bind_cairo_surface_property()

st/texture-cache: Return a GIcon from bind_cairo_surface_property()

We still load the surface into an StImageContent, but instead of
adding the content to an actor we hand out, return the content
itself (as GIcon).

That means we lose the ability to specify an icon size, but as we
get the pixel data from a fixed-size surface anyway, that shouldn't
matter much in practice.

Not to mention that the function is only used for fallback X11 icons,
which are already shit more often than not.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1342