Optimise SharedShape.PrepareFrame by avoiding a Vector3..ctor, a Vector3.op_Addition, a Matrix.CreateTranslation and one Matrix.Multiply.
Optimise RenderFrame.AddPrimitive by combining ContentsKey and Item calls in to one TryGetValue call - halving the GetHashCode() and Equals calls from the Dictionary.
Result: up to 6% faster update loop.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@459 a9f3cfff-6c41-7446-b0ca-052bd761e6eb