Number of vertices used by the index buffer of this sub-mesh.
Together with firstVertex, this defines the range of vertices that are possibly used by this sub-mesh.
You must set the vertex count so that firstVertex+vertexCount encompasses the whole range
of vertices referenced by the index buffer. For example, if the index buffer contains vertex indices 5,6,100,
then the firstVertex needs to be 5, while vertexCount needs to be 96 (100-5+1).
These values are important for any CPU-based mesh processing, 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.
Additional resources: firstVertex.