Implement window::frames subscription

... and use it in the `solar_system` example 🎉

native/src/window.rs

@@ -8,3 +8,18 @@ pub use action::Action;
 pub use event::Event;
 pub use mode::Mode;
 pub use user_attention::UserAttention;
+
+use crate::subscription::{self, Subscription};
+
+use std::time::Instant;
+
+/// Subscribes to the frames of the window of the running application.
+///
+/// The resulting [`Subscription`] will produce items at a rate equal to the
+/// framerate of the monitor of said window.
+pub fn frames() -> Subscription<Instant> {
+    subscription::raw_events(|event, _status| match event {
+        crate::Event::Window(Event::RedrawRequested(at)) => Some(at),
+        _ => None,
+    })
+}