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

[pelle.johnsen at gmail.com]

I've created http://codereview.appspot.com/4636051/, where I've tried to
extract the 'export selected' part. I haven't done extensive testing but
so far it seams to work :)

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