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

[pelle.johnsen at gmail.com]

Hi,

Let me know if you need some help with the normals stuff. It wasn't that
big a change, but the patch has grown quite large. If it's easier I can
help getting the normals fix in on top of your stuff instead of trying
to merge the whole patchset.

http://codereview.appspot.com/4387052/