Move the materials and textures lists into Mu.

Where they really belong. The reading is a bit awkward conceptually, but at
least now they're not buried in an object.
Set the texture type correctly.

Or at least more so: based on bumpmap or other.
Don't write tangents for mesh colliders.

They're not in normal .mu files, so obviously not needed.
Don't split mesh sections into submeshes.

This is what broke the exported mk1 pod. I suspect KSP/unity supports
multi-material meshes.