Vertex precision

[Philippe Hurbain]

Let's make the spec reflect what we want. What I'm reading is:
- >5dp -> error
- 5dp -> ok for Prims and Type 1 lines
- >3dp Non-Prim Type 2-5 lines -> Warning

I'm ok with this.

Any dissenters?

I'd say
- >4dp Non-Prim Type 2-5 lines -> Warning
Otherwise we'll see a gazillion of warnings for 3d patterns that require 4dp