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

[official.address243 at googlemail.com]

I integrated that patch into:
http://codereview.appspot.com/4425084/

I try to apply your review comments. Thanks. Comment on failure in
"dm->getFaceDataArray" was a big help!

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