Add a debug UI to the runtime

[freezy]

We would like to have some sort of UI during gameplay that:

1. Displays interesting stats
2. Allows interaction with the game

It should be possible to exclude this from production builds to avoid unnecessary overhead, so it's probably best to have this shipped as a separated Unity package. @ravarcade is already working on a ImgGUI implementation.

For point 1), the debug UI package needs to plug itself into somewhere in order to receive the necessary data for the stats. For point 2), it has the entire VPE API at its disposal and doesn't need any plugging.