Speed Up by only calculating entropy of cells adjacent to collapsed cells (+ naive backtracking demo) #38

// if ZERO adjacent cells are collapsed, skip for now...
        let at_least_one_collapsed = false;
        for(neighbor of [
          j > 0 ? grid[i + (j - 1) * DIM] : null, // up
          i < DIM - 1 ? grid[i + 1 + j * DIM] : null, // right
          j < DIM - 1 ? grid[i + (j + 1) * DIM] : null, // down
          i > 0 ? grid[i - 1 + j * DIM] : null // left
        ]){
            if(neighbor?.collapsed){
              at_least_one_collapsed = true;
            }
        }
        if(!at_least_one_collapsed){
          nextGrid[index] = new Cell(tiles.length, index); //grid[index];
          continue;
        }