Flood fill algorithm from: https://www.algorithm-archive.org/contents/flood_fill/flood_fill.html.

Need to explicitly define the start position, it determines which side of the boundary in the matrix will be filled.

• Example

  input matrix:

    {
    {0, 0, 0, 1, 0},
    {0, 0, 1, 0, 0},
    {0, 0, 1, 0, 0},
    {0, 0, 0, 1, 0},
    {0, 0, 0, 1, 0}
    }        
  

  The boundary consists of 1.

  -> Fill the left side:

  set start_left_pos to (0, 2) - - an arbitrary position on the left side (NOT on the boundary)

  The solution matrix (with left side filled):

    {
    {1, 1, 1, 1, 0},
    {1, 1, 1, 0, 0},
    {1, 1, 1, 0, 0},
    {1, 1, 1, 1, 0},
    {1, 1, 1, 1, 0}
    }
  

  -> Fill the right side:

  set start_right_pos to (1, 3) - - an arbitrary position on the right side (NOT on the boundary)

  The solution matrix (with right side filled):

    {
    {0, 0, 0, 1, 1},
    {0, 0, 1, 1, 1},
    {0, 0, 1, 1, 1},
    {0, 0, 0, 1, 1},
    {0, 0, 0, 1, 1}
    }