Inpaint fails when there is nothing to inpaint

[Vitaly Shukela]

Describe the bug

If inpaint mask is solid white, texture-synthesis just freezes (CPU busy) before showing any progress.

To Reproduce

$ convert -size 100x100 xc:#FFFFFF white.png
$ texture-synthesis --out-size=100x100 --in-size=100x100 --out=w.png --inpaint=white.png generate white.png 
^C

Expected behavior

A picture is generated, probably also fully white.

[Vitaly Shukela]

This picture works, but if paint a black pixel white, it stops working:

[Vitaly Shukela]

Also seems to be fixed by #26.