Add a debug option to render Z values instead of pixel colour

[MNOrac]

It would be much easier to debug issues with things like terrain Z data and shp ZAdjust / ZShapePointMove / NormalZAdjust / and so on if the game could be set to render the Z values to the screen instead of rendering the regular colour channel.

Many more modern engines allow you to set the render output to display the Z buffer and/or provide visualisation modes for many stages of the render pipeline. If something like this is possible in the TS engine, I would find it quite useful.