Add clipboard image paste support (Event::PasteImage) (#8472)

Adds `egui::Event::PasteImage`, emitted on Ctrl+V/Cmd+V when the
clipboard holds an
image with no usable text representation (mirrors
`OutputCommand::CopyImage` for
the opposite direction). `egui-winit::Clipboard` gains `get_image()`,
backed by
`arboard::Clipboard::get_image()` — the symmetric counterpart to the
`set_image()`
already used for copy. Wired in the winit keyboard-shortcut path and in
both the
glow and wgpu eframe backends' AccessKit-driven paste action.

Straight (unmultiplied) alpha from the OS clipboard is converted via the
existing
`ColorImage::from_rgba_unmultiplied` helper rather than reinterpreting
bytes
directly, since the two aren't bit-compatible for translucent pixels.

Not covered here: the web (wasm32) target — browser clipboard image
reads are
inherently async (`Blob::array_buffer` returns a Promise), while
`Event::PasteImage`
as designed here carries an already-decoded `ColorImage`. Bridging that
needs
either an async event payload or a deferred/queued event on `AppRunner`
— more of
a design decision than a small mirrored addition, flagging it rather
than guessing.

* Related: #2108 (not fixed: web)

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/native/glow_integration.rs

@@ -831,6 +831,11 @@ impl GlowWinitRunning<'_> {
                                         .events
                                         .push(egui::Event::Paste(contents));
                                 }
+                            } else if let Some(image) = egui_winit.clipboard_image() {
+                                egui_winit
+                                    .egui_input_mut()
+                                    .events
+                                    .push(egui::Event::PasteImage(std::sync::Arc::new(image)));
                             }
                         }
                     }

crates/eframe/src/native/wgpu_integration.rs

@@ -838,6 +838,11 @@ impl WgpuWinitRunning<'_> {
                                     .events
                                     .push(egui::Event::Paste(contents));
                             }
+                        } else if let Some(image) = egui_winit.clipboard_image() {
+                            egui_winit
+                                .egui_input_mut()
+                                .events
+                                .push(egui::Event::PasteImage(std::sync::Arc::new(image)));
                         }
                     }
                 }