Add a public constructor to OsdMesh

Added a public constructor to OsdMesh that initializes the member objects from its arguments, allowing those members to be created by a subclass or an external function.

* maintainance work on the D3D11 specialization of OsdMesh to bring it in line with the other template specializations
* updated the facePartition example to derive PartitionedMesh from OsdMesh in order to allow other vertex buffer and compute controller configurations