Extract the vertex indices located at the corners of a mesh facet.

Input parameters:

1. Fa → Mesh facet to extract indices from.

Output parameters:

1. A → Vertex index at first corner.
2. B → Vertex index at second corner.
3. C → Vertex index at third corner.
4. D → Vertex index at fourth corner. If the facet is a triangle, the fourth index will be equal to the third index.