Improve rendering

[HagaiHargil]

The rendering status is in a type of a local maximum - it works fine and looks OKish, but in the long term it will simply not be sufficient.

The underlying library (kiss3d) is unmaintained, and some of the functionality it provides is broken. Moreover, it can only display a single window at any given moment, which means that we currently only render the merged channel.

It seems that the best way forward is to switch the underlying library, but the options are quite limited - there are currently no other high-level GPU-based rendering libs in Rust. Bevy and rg3d might go there one day, but currently they're not trying to. The only option is to go down the stack and re-implement a subset of kiss3d's functionality using the dependencies of kiss3d.