Implement Debug for TriMesh

[alice-i-cecile]

This would be useful to quickly validate if a TriMesh looks as expected (and to allow derives on types containing trimeshes).

The naive derived Debug impl is obviously unhelpful. Instead, perhaps report:

• The number of triangles
• The AABB
• Any flags
• The topology information, if present

[whatf0xx]

Hi! I was actually looking at something like this recently and unsure of what a useful Debug implementation would look like for TriMesh. I will try and take a look at this with the information you suggested and get back to you. :)