// Numbering scheme for the cube of faces of panels of cells ...
  //
  //                 +-------+-------+
  //                 | 0   1 | 0   1 |
  //                 |   0   |   1   |
  //                 | 2   3 | 2   3 |
  //                 +------ 0 ------+
  //                 | 0   1 | 0   1 |
  //                 |   2   |   3   |
  //                 | 2   3 | 2   3 |
  // +-------+-------+-------+-------+-------+-------+-------+-------+
  // | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 |
  // |   0   |   1   |   0   |   1   |   0   |   1   |   0   |   1   |
  // | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 |
  // +------ 1 ------+------ 2 ------+------ 3 ------+------ 4 ------+
  // | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 | 0   1 |
  // |   2   |   3   |   2   |   3   |   2   |   3   |   2   |   3   |
  // | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 | 2   3 |
  // +-------+-------+-------+-------+-------+-------+-------+-------+
  //                 | 0   1 | 0   1 |
  //                 |   0   |   1   |
  //                 | 2   3 | 2   3 |
  //                 +------ 5 ------+
  //                 | 0   1 | 0   1 |
  //                 |   2   |   3   |
  //                 | 2   3 | 2   3 |
  //                 +-------+-------+
  //
  // a cell is a single LED
  // a panel is a set of four cells
  // a face is a set of four panels
  // the cube is a set of six faces