Resetting of index and vertex data implemented

In the dispose call the index and vertex data is now reset. When disposed before this would never decrease the index and vertex data so if a reference to the QuadBatch still existed (for example being reused in RenderSupport) then unnecessary data could hang around and cause a build up of memory.

Clearing this should not be too expensive performance wise as dispose is not called that often on QuadBatches in general.

Also I nulled out the index and vertex buffers as it was possible that dispose could be called on them when they were already disposed