Use linear image iterator

[Horki]

Use linear image iterator

[Horki]

@silvia-odwyer before you close ticket, could you also update https://github.com/silvia-odwyer/photon/wiki/Benchmarks

[Horki]

@silvia-odwyer #37 please check

[Horki]

@silvia-odwyer just saw this, https://github.com/image-rs/image/blob/master/src/image.rs#L638-#L651, so I can do a new branch, and remove my custom iterator, which is redundant

[Horki]

@silvia-odwyer ok, I have tried, and IMHO, my iterator is very naive, but still better solution than O(N^2) loop