yoda: gate iced_wgpu x11 backend behind an opt-in feature

iced_wgpu's unix block unconditionally depended on as-raw-xcb-connection,
tiny-xlib and x11rb for a DRI3 device-id fallback in
window/compositor.rs that only fires when wayland can't determine the
GPU. On a Wayland-only build this code path never runs, so the deps
are pure bloat.

Changes:
- Cargo.toml: move the 3 x11 crates to optional + new feature `x11`
  (not in default — wayland-only is the yoda target; users who need
  X11 opt in).
- window.rs: cfg-gate `mod x11;` on feature = "x11"
- window/compositor.rs: cfg-gate `use super::x11::get_x11_device_ids`
  + the or_else fallback on feature = "x11" (wrapped in a let-binding
  so the non-x11 branch compiles cleanly).

Result: iced_wgpu no longer pulls any x11 crate when built without
the feature.

wgpu/src/window.rs

@@ -7,7 +7,7 @@ pub mod compositor;
     not(target_os = "redox")
 ))]
 mod wayland;
-#[cfg(all(unix, not(target_os = "macos"), not(target_os = "redox")))]
+#[cfg(all(unix, feature = "x11", not(target_os = "macos"), not(target_os = "redox")))]
 mod x11;
 
 pub use compositor::Compositor;