Expose compositor from subcompositor

[Friz64]

This PR implements one way of exposing a WlCompositor from a SubcompositorState.

To avoid a potential XY problem, here's my situation:

I want to change the input region of a (sub)surface of a wayland frame. I only have access to a SubcompositorState. To change the input region, I need to create new regions that I can pass into set_input_region. SCTK only allows the creation of new regions through a compositor, but not through a subcompositor.

See:

• Window shadows PolyMeilex/sctk-adwaita#43

Is exposing the compositor from the subcompositor the correct way of going about this, and if so, is my solution good?

By implementing ProvidesBoundGlobal, I can leverage the Region abstraction from SCTK: https://docs.rs/smithay-client-toolkit/latest/smithay_client_toolkit/compositor/struct.Region.html#method.new

[Friz64]

I only have access to a SubcompositorState.

Well, the frame's constructor only takes a SubcompositorState, but it could theoretically also take a CompositorState. I'm not too sure if this would be the best solution though, as it feels clunky (e.g. winit would need to add another special case here: https://github.com/rust-windowing/winit/blob/e9210555c187c0573b1d13d44fa83035ef3323b0/src/platform_impl/linux/wayland/window/state.rs#L273-L280).

[Friz64]

I only have access to a SubcompositorState.

Well, the frame's constructor only takes a SubcompositorState, but it could theoretically also take a CompositorState. I'm not too sure if this would be the best solution though, as it feels clunky (e.g. winit would need to add another special case here: https://github.com/rust-windowing/winit/blob/e9210555c187c0573b1d13d44fa83035ef3323b0/src/platform_impl/linux/wayland/window/state.rs#L273-L280).

I guess adding a special case is fine.