x11: fix positioning with --position arg

The child window should always have position 0,0.

This was a regression from commit 809bcc55.

Fixes #5395.

window/src/os/x11/window.rs

@@ -1435,8 +1435,8 @@ impl XWindow {
                 depth: conn.depth,
                 wid: child_id,
                 parent: window_id,
-                x: x.unwrap_or(0).try_into()?,
-                y: y.unwrap_or(0).try_into()?,
+                x: 0,
+                y: 0,
                 width: width.try_into()?,
                 height: height.try_into()?,
                 border_width: 0,