event_loop: honour App::window_size_hint() on the first configure

Previously only `set_min_size()` was called with the requested size, on the assumption that the compositor would adopt it as the initial dimension. In practice that doesn't hold: xdg-shell has no "preferred initial size" primitive and compositors are free to pick any size within `[min, max]` on the first configure. The window ended up opening at the 800x600 fallback instead of the size the application had asked for.
The fix pins `min == max` before the first commit, which forces the compositor to honour the requested size in its first configure, and then releases `max_size` from the configure handler via the `pending_size_hint_unpin` latch so the surface remains user-resizable afterwards. The 800x600 fallback now only applies when the application does not provide a hint.

src/event_loop/app_data.rs

@@ -675,6 +675,9 @@ pub struct AppData<A: App>
 	pub exit_requested:      bool,
 	/// First-configure latch for `App::start_fullscreen`.
 	pub pending_fullscreen:  bool,
+	/// First-configure latch: clears `max_size` after the compositor
+	/// has honoured the pinned initial size.
+	pub pending_size_hint_unpin: bool,
 	/// Main application surface.
 	pub main:                SurfaceState<A::Message>,
 	/// Auxiliary layer-shell surfaces keyed by their stable [`OverlayId`].