x11: fix WindowAttributesExtX11::with_x11_screen()

Based on https://github.com/rust-windowing/winit/pull/3973, which should
be merged first.

There's an API to programmatically specify X11 screen id (override what
is determined from the `DISPLAY` env variable), but it doesn't work.

Seeting up X Server with 2 screens and calling `DISPLAY=:0
X11_SCREEN_ID=1 cargo run --example window` should be equivalent to
calling `DISPLAY=:0.1 cargo run --example window`

The latter works (and places the window on the correct screen), but the
former yields

`failed to create initial window: Os(OsError { line: 620, file:
"src/platform_impl/linux/x11/window.rs", error: X11Error(X11Error {
error_kind: Match, error_code: 8, sequence: 219, bad_value: 1319,
minor_opcode: 0, major_opcode: 1, extension_name: None, request_name:
Some("CreateWindow") }) })`

_Here `1319` is the root window id for screen 0, which doesn't match the
screen 1 that we request._

The problem is that we need to factor in the screen id when determining
the parent (root) window when not explicitly set. This patch does that.

---

Also: Extend the window example with X11_{SCREEN,VISUAL}_ID env variables

src/changelog/unreleased.md

@@ -44,6 +44,8 @@ changelog entry.
 
 - On macOS, add `WindowExtMacOS::set_borderless_game` and `WindowAttributesExtMacOS::with_borderless_game`
   to fully disable the menu bar and dock in Borderless Fullscreen as commonly done in games.
+- On X11, the `window` example now understands the `X11_VISUAL_ID` and `X11_SCREEN_ID` env
+  variables to test the respective modifiers of window creation.
 
 ### Fixed
 
@@ -54,3 +56,4 @@ changelog entry.
 - On X11, key events forward to IME anyway, even when it's disabled.
 - On Windows, make `ControlFlow::WaitUntil` work more precisely using `CREATE_WAITABLE_TIMER_HIGH_RESOLUTION`.
 - On X11, creating windows on screen that is not the first one (e.g. `DISPLAY=:0.1`) works again.
+- On X11, creating windows while passing `with_x11_screen(non_default_screen)` works again.

src/platform_impl/linux/x11/window.rs

@@ -144,12 +144,26 @@ impl UnownedWindow {
     ) -> Result<UnownedWindow, RootOsError> {
         let xconn = &event_loop.xconn;
         let atoms = xconn.atoms();
+
+        let screen_id = match window_attrs.platform_specific.x11.screen_id {
+            Some(id) => id,
+            None => xconn.default_screen_index() as c_int,
+        };
+
+        let screen = {
+            let screen_id_usize = usize::try_from(screen_id)
+                .map_err(|_| os_error!(OsError::Misc("screen id must be non-negative")))?;
+            xconn.xcb_connection().setup().roots.get(screen_id_usize).ok_or(os_error!(
+                OsError::Misc("requested screen id not present in server's response")
+            ))?
+        };
+
         #[cfg(feature = "rwh_06")]
         let root = match window_attrs.parent_window.as_ref().map(|handle| handle.0) {
             Some(rwh_06::RawWindowHandle::Xlib(handle)) => handle.window as xproto::Window,
             Some(rwh_06::RawWindowHandle::Xcb(handle)) => handle.window.get(),
             Some(raw) => unreachable!("Invalid raw window handle {raw:?} on X11"),
-            None => event_loop.root,
+            None => screen.root,
         };
         #[cfg(not(feature = "rwh_06"))]
         let root = event_loop.root;
@@ -207,19 +221,6 @@ impl UnownedWindow {
             dimensions
         };
 
-        let screen_id = match window_attrs.platform_specific.x11.screen_id {
-            Some(id) => id,
-            None => xconn.default_screen_index() as c_int,
-        };
-
-        let screen = {
-            let screen_id_usize = usize::try_from(screen_id)
-                .map_err(|_| os_error!(OsError::Misc("screen id must be non-negative")))?;
-            xconn.xcb_connection().setup().roots.get(screen_id_usize).ok_or(os_error!(
-                OsError::Misc("requested screen id not present in server's response")
-            ))?
-        };
-
         // An iterator over the visuals matching screen id combined with their depths.
         let mut all_visuals = screen
             .allowed_depths