support GTK graphics offload

[benjamin-otte]

GTK 4.14 gained support for graphics offloading, see https://blog.gtk.org/2023/11/15/introducing-graphics-offload/ and it would be nice if Glide could make use of it.

It'd also be nice because https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1422 needs finishing and an application actually making use of that would be neat. 😉

[philn]

Seems like @sdroege is on it ;)

[sdroege]

Yeah, @benjamin-otte agreed to fix everything that I find broken in GTK related to that so it shouldn't be much work for me!