Wireframe "last position" view for debugging

[dany-on-demand]

From: yvt/openspades#746

It would be nice to have (ideally gated via compile time flag so people don't feel compelled to cheat) a view that shows the last sent and received position of the player, ideally as colored wireframes.
This would be very useful for debugging the server prediction code.

For rendering - just enable wireframe mode on kv6 render, if voxlap mode is not enabled, and disable on exit. For previous positions, just make a variable array, I don't expect devs would run this for so long that they'd exhaust available ram lol

I agree that this should be hidden behind a compile flag. Maybe DEV_OR_ADMIN_TOOLS since this would also be useful for admins?