Allow attaching custom user data to a screenshot command (#5416)

This lets users trigger a screenshot from anywhere, and then when they
get back the results they have some context about what part of their
code triggered the screenshot.

crates/eframe/src/native/glow_integration.rs

@@ -661,13 +661,14 @@ impl<'app> GlowWinitRunning<'app> {
         {
             for action in viewport.actions_requested.drain() {
                 match action {
-                    ActionRequested::Screenshot => {
+                    ActionRequested::Screenshot(user_data) => {
                         let screenshot = painter.read_screen_rgba(screen_size_in_pixels);
                         egui_winit
                             .egui_input_mut()
                             .events
                             .push(egui::Event::Screenshot {
                                 viewport_id,
+                                user_data,
                                 image: screenshot.into(),
                             });
                     }