Should merge Vertex Buffers and Index Buffers

[walbourn]

The tool currently emits one VB and IB per mesh, with the subsets sharing the same IB/VB. For scenes with lots of meshes, this can result in a lot of potentially small buffers.

There should be an optimization flag added to the tool which will merge/coalesce VBs/IBs into fewer shared buffers. In theory each VB could have a different input layout, so that should be taken into account.