Index count for this sub-mesh face data.
This property, together with indexStart, determines which part of the whole index buffer is used by
a particular sub-mesh. Values in the index buffer are interpreted according to the topology property,
for example the usual MeshTopology.Triangles topology needs three indices for each triangle.
Additional resources: indexStart, Mesh.SetSubMesh, Mesh.GetSubMesh, Mesh.SetIndexBufferParams, Mesh.SetIndexBufferData.