Add OutputCommand::SetPointerPosition to set mouse position (#5776)

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This is not supported on the web and not yet on Wayland.

~~I also had to update `ring` and add an exception for `paste` being
unmaintained.~~ Has since been updated on master.

* [x] I have followed the instructions in the PR template

crates/egui_demo_lib/src/demo/tests/cursor_test.rs

@@ -16,6 +16,14 @@ impl crate::Demo for CursorTest {
 
 impl crate::View for CursorTest {
     fn ui(&mut self, ui: &mut egui::Ui) {
+        if ui
+            .button("Center pointer in window")
+            .on_hover_text("The platform may not support this.")
+            .clicked()
+        {
+            let position = ui.ctx().available_rect().center();
+            ui.ctx().set_pointer_position(position);
+        }
         ui.vertical_centered_justified(|ui| {
             ui.heading("Hover to switch cursor icon:");
             for &cursor_icon in &egui::CursorIcon::ALL {