More lights

[ffreyer]

Except for RPRMakie we can currently only handle one light source and that one light source is always a point light with infinite range. I've seen plenty of questions about adding more light sources though I don't see any formal requests on github.

The simplest extension would be to collect light attributes in a struct and allow a vector of lights to passed. I don't think glsl can handle dynamic arrays so the number of lights would be shader compile time constant. A nice addition would be the ability to recompile shaders when such constants change.

After that we can look towards forward+ (or deferred) rendering to increase the amount of lights we can handle. https://www.3dgep.com/forward-plus/ looks like a good resource for all of this.