winit/wayland: Subsurfaces for drag surfaces

Use `Icon::Surface` instead of `Icon::Buffer`, so we can then create
subsurfaces of the `wl_surface`.

Using `.screenshot()` then copying to an shm buffer is suboptimal, but
this does seem overall better than with the older Iced version when a
drag surface didn't appear until a Vulkan surface could be created for
it.

This re-uses `Connection` in the platform-specific code, instead of
creating from display handle on each call.

Cargo.toml

@@ -296,6 +296,7 @@ wgpu = { version = "27.0", default-features = false, features = [
     "wgsl",
 ] }
 wayland-protocols = { version = "0.32.1", features = ["staging"] }
+wayland-client = { version = "0.31.5" }
 # web-time = "1.1"