Wayland: add WindowAttributesWayland::with_csd_fallback to opt out of client-side decoration fallback

[benedikt-weyer]

Summary

On Wayland, when a window requests server-side decoration but the
compositor responds DecorationMode::Client, winit always falls back to
drawing its own client-side decoration frame (AdwaitaFrame/FallbackFrame).

Some applications draw no titlebar/CSD of their own and want the
compositor's xdg-decoration negotiation to be the sole authority over
whether the window is decorated at all — for them, the automatic CSD
fallback means an unwanted title bar appears even when the compositor
intentionally declines to decorate (e.g. a compositor exposing a
user-level toggle for its own server-side decoration).

This adds WindowAttributesWayland::with_csd_fallback(bool), defaulting to
true (today's behavior, so this is non-breaking). Setting it to false
skips creating the fallback frame entirely: a Client decoration-mode
response then simply means the window has no decoration, instead of
triggering winit's own drawn one.

Changes

• winit-wayland/src/lib.rs: new WindowAttributesWayland::csd_fallback
  field (default true) and with_csd_fallback builder method, alongside
  the existing prefer_csd field/builder.
• winit-wayland/src/window/{mod.rs,state.rs}: thread csd_fallback
  through to WindowState, mirroring how prefer_csd is threaded today.
• winit-wayland/src/window/state/configure.rs: AND self.csd_fallback
  into the existing fallback-frame-creation condition.
• winit-wayland/src/popup.rs: pass true for popups (unaffected either
  way, since popups never create this frame — kept for signature parity).
• winit/src/changelog/unreleased.md: changelog entry.

Test plan

• cargo check -p winit-wayland --features sctk-adwaita
• cargo check -p winit --features x11,wayland
• cargo +nightly fmt -p winit-wayland -- --check
• cargo +nightly fmt -- --check (whole workspace)

---

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

🤖 Generated with Claude Code