Diagnostics overlay

TODO: Add a screenshot of the DiagnosticsOverlay in a real app.

Bevy's diagnostics have always been easy to dump to the terminal, but displaying them in-game meant wiring up your own UI. DiagnosticsOverlayPlugin adds a built-in overlay for this with presets for common cases:

commands.spawn(DiagnosticsOverlay::fps());
commands.spawn(DiagnosticsOverlay::mesh_and_standard_materials());

You can also build a custom overlay from any [DiagnosticPath] list:

commands.spawn(DiagnosticsOverlay::new("MyDiagnostics", vec![MyDiagnostics::COUNTER.into()]));

By default, the overlay shows a smoothed moving average. You can show the latest value or the raw moving average instead using DiagnosticsOverlayItem::statistic and configure floating-point precision with DiagnosticsOverlayItem::precision:

commands.spawn(DiagnosticsOverlay::new("MyDiagnostics", vec![DiagnosticsOverlayItem {
    path: MyDiagnostics::COUNTER,
    statistic: DiagnosticsOverlayStatistic::Value,
    precision: 4,
}]));

Check out the updated [scene_viewer example] to see it in action!