Lag seems to cause key release events to be missed

## Bevy version

```
[[package]]
name = "bevy"
version = "0.7.0"
```

## Operating system & version

Debian Linux, Wheezy

## What you did

Ran a debug build of `alien_cake_addict` on my ancient gtx 980 (760?). Tried to play.

## What happened

The framerate was low (maybe around 20fps). The keys tended to get stuck, in a sense, I would run in one direction for a bit, I would release the key, I'd press other directions, but the alien would not stop running in that direction, could not be turned. It would eventually stop pushing in that direction, seemingly at random, usually after the nearest cake had been tragically missed.

Oddly, the game also seemed to be harder with the lag (as if there were was less time to get each cake)

I can't think of a reason this amount of lag should cause this, but it seems to, and that makes me worry that sometimes it might miss key state changes at a normal framerate too, or during a lag spike.

## Additional Information

Sometimes when my entire system is lagging, key repeats will occur. I wouldn't expect this to be the same issue though.