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/src/context.rs

@@ -1482,6 +1482,11 @@ impl Context {
         self.send_cmd(crate::OutputCommand::CopyImage(image));
     }
 
+    /// Set the mouse cursor position (if the platform supports it).
+    pub fn set_pointer_position(&self, position: Pos2) {
+        self.send_cmd(crate::OutputCommand::SetPointerPosition(position));
+    }
+
     /// Format the given shortcut in a human-readable way (e.g. `Ctrl+Shift+X`).
     ///
     /// Can be used to get the text for [`crate::Button::shortcut_text`].

crates/egui/src/data/output.rs

@@ -95,6 +95,9 @@ pub enum OutputCommand {
 
     /// Open this url in a browser.
     OpenUrl(OpenUrl),
+
+    /// Set the mouse cursor position (if the platform supports it).
+    SetPointerPosition(emath::Pos2),
 }
 
 /// The non-rendering part of what egui emits each frame.