Fixed a bug in FBX Exporter that prevented SkinnedModels (animated rigs) from working

The FBX Exporter was causing newer rigged & animated models in Autodesk Maya to lose their animations.  Older models and Microsoft's Dude worked fine.  Upon close inspection, I discovered that Maya was consistently returning a vertex stride >=52, not >=64, for skinned models (models w/ vertices that have PositionNormalTexturedWeight).

Thus, changed Vertex Stride from 64 to 52 in Exporters/XNA - OBJ/BabylonExport.Core/Entities/PositionNormalTexturedWeights.cs