First vertex in the index buffer for this sub-mesh.
Together with vertexCount, this defines the range of vertices that are possibly used by this sub-mesh.
These values are important for any CPU-based mesh processing that would be done, for example
dynamic batching or CPU skinning.
The bounds, firstVertex and vertexCount values are calculated automatically by Mesh.SetSubMesh, unless
MeshUpdateFlags.DontRecalculateBounds flag is passed.
See Also: vertexCount.