Add Context::copy_image (#5533)

* Closes https://github.com/emilk/egui/issues/5424

This adds support for copying images to the system clipboard on native
and on web using `Context::copy_image`.

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -92,6 +92,7 @@ impl Default for DemoGroups {
                 Box::<super::window_options::WindowOptions>::default(),
             ]),
             tests: DemoGroup::new(vec![
+                Box::<super::tests::ClipboardTest>::default(),
                 Box::<super::tests::CursorTest>::default(),
                 Box::<super::tests::GridTest>::default(),
                 Box::<super::tests::IdTest>::default(),

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

@@ -0,0 +1,81 @@
+pub struct ClipboardTest {
+    text: String,
+}
+
+impl Default for ClipboardTest {
+    fn default() -> Self {
+        Self {
+            text: "Example text you can copy-and-paste".to_owned(),
+        }
+    }
+}
+
+impl crate::Demo for ClipboardTest {
+    fn name(&self) -> &'static str {
+        "Clipboard Test"
+    }
+
+    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+        egui::Window::new(self.name()).open(open).show(ctx, |ui| {
+            use crate::View as _;
+            self.ui(ui);
+        });
+    }
+}
+
+impl crate::View for ClipboardTest {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.label("egui integrates with the system clipboard.");
+        ui.label("Try copy-cut-pasting text in the text edit below.");
+
+        let text_edit_response = ui
+            .horizontal(|ui| {
+                let text_edit_response = ui.text_edit_singleline(&mut self.text);
+                if ui.button("📋").clicked() {
+                    ui.ctx().copy_text(self.text.clone());
+                }
+                text_edit_response
+            })
+            .inner;
+
+        if !cfg!(target_arch = "wasm32") {
+            // These commands are not yet implemented on web
+            ui.horizontal(|ui| {
+                for (name, cmd) in [
+                    ("Copy", egui::ViewportCommand::RequestCopy),
+                    ("Cut", egui::ViewportCommand::RequestCut),
+                    ("Paste", egui::ViewportCommand::RequestPaste),
+                ] {
+                    if ui.button(name).clicked() {
+                        // Next frame we should get a copy/cut/paste-event…
+                        ui.ctx().send_viewport_cmd(cmd);
+
+                        // …that should en up here:
+                        text_edit_response.request_focus();
+                    }
+                }
+            });
+        }
+
+        ui.separator();
+
+        ui.label("You can also copy images:");
+        ui.horizontal(|ui| {
+            let image_source = egui::include_image!("../../../data/icon.png");
+            let uri = image_source.uri().unwrap().to_owned();
+            ui.image(image_source);
+
+            if let Ok(egui::load::ImagePoll::Ready { image }) =
+                ui.ctx().try_load_image(&uri, Default::default())
+            {
+                if ui.button("📋").clicked() {
+                    ui.ctx().copy_image((*image).clone());
+                }
+            }
+        });
+
+        ui.vertical_centered_justified(|ui| {
+            ui.add(crate::egui_github_link_file!());
+        });
+    }
+}

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

@@ -1,3 +1,4 @@
+mod clipboard_test;
 mod cursor_test;
 mod grid_test;
 mod id_test;
@@ -7,6 +8,7 @@ mod layout_test;
 mod manual_layout_test;
 mod window_resize_test;
 
+pub use clipboard_test::ClipboardTest;
 pub use cursor_test::CursorTest;
 pub use grid_test::GridTest;
 pub use id_test::IdTest;

crates/egui_demo_lib/src/rendering_test.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 
 use egui::{
-    emath::GuiRounding, epaint, lerp, pos2, vec2, widgets::color_picker::show_color, Align2,
+    emath::GuiRounding as _, epaint, lerp, pos2, vec2, widgets::color_picker::show_color, Align2,
     Color32, FontId, Image, Mesh, Pos2, Rect, Response, Rgba, RichText, Sense, Shape, Stroke,
     TextureHandle, TextureOptions, Ui, Vec2,
 };