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

[official.address243 at googlemail.com]

Patch on http://codereview.appspot.com/4425084/ fixes the problem with
the wrongly casted CustomData*. But now all normals are wongly exported,
all faces are flat.

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