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

@@ -70,6 +70,8 @@ changelog entry.
 - Add `PointerKind`, `PointerSource`, `ButtonSource`, `FingerId`, `primary` and `position` to all
   pointer events as part of the pointer event overhaul.
 - Add `DeviceId::into_raw()` and `from_raw()`.
+- 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.
 
 ### Changed
 
@@ -196,3 +198,4 @@ 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.