Describes the vertex's position.
Note this value is a Vector3. If the vertex represents flat UI geometry, set the z component of this position field to Vertex.nearZ. The position value is relative to the VisualElement's local rectangle top-left corner. The coordinate system is X+ to the right, and Y+ goes down. The unit of position is VisualElement points. When the vertices are indexed, the triangles described must follow clock-wise winding order given that Y+ goes down.