Zero is background option

[cooperlab]

For sparse inputs like detected cells, allow the zero value in the pixelmap to be ignored. This would allow background to be ignored during painting and prevent display of background in guided labeling.

[andsild]

Currently have a temporary solution/hack for this, using a bbox of [0,0,1,1] in this commit: 97ea16a

Haven't tested in using painting tools for annotations, though, only detected cells