PointPrimitiveCollection on terrain

[tfili]

Suffers from #3921 just like billboards do mainly because the code to implement this is almost identical.

Before this is merged we need to figure out how to fix the clipping. For billboards we force bottom vertical alignment. Maybe we can do the same here.

[mramato]

After some offline discussion, there is concern that clamping points is not a common enough operation to warrant the overhead this code would add to all points. Since the goal here is supporting clamping of Entity points, @tfili is going to look into doing that at the PointVisualizer level by using clamped billboards in those cases.

We can leave this open until that PR is ready, just in case we decide to go back to this.

[mramato]

Superseded by #3903.

I won't delete the branch right away in case we need this for something down the line.