Add InputState::stable_dt

[emilk]

This provides a better estimate of a typical frametime in reactive mode.

Summary: when animating things, stop using unstable_dt and start using stable_dt.min(0.1).

From the docstring of stable_dt:

---

Time since last frame (in seconds), but gracefully handles the first frame after sleeping in reactive mode.

In reactive mode (available in e.g. eframe), egui only updates when there is new input
or something is animating.
This can lead to large gaps of time (sleep), leading to large [Self::unstable_dt].

If egui requested a repaint the previous frame, then egui will use
stable_dt = unstable_dt;, but if egui did not not request a repaint last frame,
then egui will assume unstable_dt is too large, and will use
stable_dt = predicted_dt;.

This means that for the first frame after a sleep,
stable_dt will be a prediction of the delta-time until the next frame,
and in all other situations this will be an accurate measurement of time passed
since the previous frame.

Note that a frame can still stall for various reasons, so stable_dt can
still be unusually large in some situations.

When animating something, it is recommended that you use something like
stable_dt.min(0.1) - this will give you smooth animations when the framerate is good
(even in reactive mode), but will avoid large jumps when framerate is bad,
and will effectively slow down the animation when FPS drops below 10.