Implement support for GTK graphics offloading

[benjamin-otte]

GTK 4.14 gained support for graphics offloading, see https://blog.gtk.org/2023/11/15/introducing-graphics-offload/ It would be nice if clapper made use of that.

This probably requires porting the changes in https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6618 into the plugin and adding a https://docs.gtk.org/gtk4/class.GraphicsOffload.html widget in the right place.

[nekohayo]

Probably related: @rmader's #343