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.
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Also: Extend the window example with X11_{SCREEN,VISUAL}_ID env variables
ae4c449670