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

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Hello "dBugSlayer",
I suggest we work together. It would be great if you would apply my
patch: http://codereview.appspot.com/4425084/, and continue your work
with that patch. I fixed already some of the stuff Brecht mentioned. I
suggest we fix at first the code, so it works flawless, then going after
all wrong spaces, newlines, etc..

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