Examples consume a lot of CPU time when not visible on macOS

[parasyte]

macOS has an unusual behavior that it will not wait for V-Blank when the window is not visible. That can happen e.g. when the window is minimized or it is entirely covered by another window. While the issue is limited to examples, it's still bad because the examples are how people learn to use this crate.

[parasyte]

Just so I don't forget later, here are some examples of how this is currently handled in the wild:

• wgpu examples
• egui_glium
• veloren - The clock implementation does the actual sleep.

[parasyte]

This is fixed in the invaders example with a fixed time step and sleeping the main thread to cap the frame rate:

pixels/examples/invaders/src/main.rs

Lines 161 to 166 in 3968c97

	// Sleep the main thread to limit drawing to the fixed time step.
	// See: https://github.com/parasyte/pixels/issues/174
	let dt = TIME_STEP.as_secs_f64() - Time::now().sub(&g.current_instant());
	if dt > 0.0 {
	std::thread::sleep(Duration::from_secs_f64(dt));
	}