Support epaint::Shape::Callback(PaintCallback)

[PPakalns]

bevy_egui/src/egui_node.rs

Line 222 in 6705757

unimplemented!("Paint callbacks aren't supported")

PaintCallback is not supported. This callback allows painting 3D views, visualizations, etc inside egui Windows using underlying backend (bevy rendering / wgpu).

Maybe PaintCallback could somehow get expanded into render graph subgraph.
https://docs.rs/bevy_render/latest/bevy_render/render_graph/struct.RenderGraph.html#method.add_sub_graph

[mvlabat]

I'm happy to accept a PR implementing this. Unfortunately, I don't have enough capacity to work on this myself.