Replace eframe::Frame commands and WindowInfo with egui (#3564)

* Part of https://github.com/emilk/egui/issues/3556

## In short
You now almost never need to use `eframe::Frame` - instead use
`ui.input(|i| i.viewport())` for information about the current viewport
(native window), and use `ctx.send_viewport_cmd` to modify it.

## In detail

This PR removes most commands from `eframe::Frame`, and replaces them
with `ViewportCommand`.
So `frame.close()` becomes
`ctx.send_viewport_cmd(ViewportCommand::Close)`, etc.

`frame.info().window_info` is now also gone, replaced with `ui.input(|i|
i.viewport())`.

`frame.info().native_pixels_per_point` is replaced with `ui.input(|i|
i.raw.native_pixels_per_point)`.

`RawInput` now contains one `ViewportInfo` for each viewport.

Screenshots are taken with
`ctx.send_viewport_cmd(ViewportCommand::Screenshots)` and are returned
in `egui::Event` which you can check with:

``` ust
ui.input(|i| {
    for event in &i.raw.events {
        if let egui::Event::Screenshot { viewport_id, image } = event {
            // handle it here
        }
    }
});
```

### Motivation
You no longer need to pass around the `&eframe::Frame` everywhere.
This also opens the door for other integrations to use the same API of
`ViewportCommand`s.

crates/egui_glow/src/winit.rs

@@ -12,6 +12,8 @@ pub struct EguiGlow {
     pub egui_winit: egui_winit::State,
     pub painter: crate::Painter,
 
+    viewport_info: egui::ViewportInfo,
+
     // output from the last update:
     shapes: Vec<egui::epaint::ClippedShape>,
     pixels_per_point: f32,
@@ -43,6 +45,7 @@ impl EguiGlow {
             egui_ctx: Default::default(),
             egui_winit,
             painter,
+            viewport_info: Default::default(),
             shapes: Default::default(),
             pixels_per_point,
             textures_delta: Default::default(),
@@ -50,7 +53,8 @@ impl EguiGlow {
     }
 
     pub fn on_event(&mut self, event: &winit::event::WindowEvent<'_>) -> EventResponse {
-        self.egui_winit.on_event(&self.egui_ctx, event)
+        self.egui_winit
+            .on_event(&self.egui_ctx, event, ViewportId::ROOT)
     }
 
     /// Call [`Self::paint`] later to paint.
@@ -71,7 +75,17 @@ impl EguiGlow {
             log::warn!("Multiple viewports not yet supported by EguiGlow");
         }
         for (_, ViewportOutput { commands, .. }) in viewport_output {
-            egui_winit::process_viewport_commands(commands, window, true);
+            let mut screenshot_requested = false;
+            egui_winit::process_viewport_commands(
+                &mut self.viewport_info,
+                commands,
+                window,
+                true,
+                &mut screenshot_requested,
+            );
+            if screenshot_requested {
+                log::warn!("Screenshot not yet supported by EguiGlow");
+            }
         }
 
         self.egui_winit.handle_platform_output(