[Bf-codereview] Use DerivedMesh instead of Mesh in COLLADA GeometryExporter (issue4387052)

[Daniel Tavares]

I'll update my changes to be based off of what you have on Codereivew.
That way it's easier to see what changes I've made.

I haven't tested anything related to UVs or vertex colors. I'd assume
they're broken ATM.