code: function CollisionMatrix(x, y, r) {
      this.resolution = r;
      this.xc = x;
      this.yc = y;
      this.matrix = new Array(x);
      for (var i = 0; i <= (x + 5); i++) {
          this.matrix[i] = new Array(y);
          for (var j = 0; j <= (y + 5); ++j) {
              this.matrix[i][j] = new DropItem(null);
          }
      }
  }
docs: Defines a gravity matrix object which handles collision detection.
  @param x number of columns in the matrix
  @param y number of rows in the matrix
  @param r grid size