"Fill Label Holes" plugin results in full-white image when applied to binary image

[dlegland]

When the "Fill Label Holes" plugin is applied to a binary image (or to an image with only one label except the background), the result is an image totally filled with the label value.

Possible solution: after CC-labeling of BG regions is computed, discard the BG regions that touch the border of the image.

Should also return the result in a new image.