eval_semantic_segmentation and calc_semantic_segmentation_confusion for when we have ignore label

[seyeeet]

it would be nice if we can add an option to eval_semantic_segmentation and calc_semantic_segmentation_confusion for the cases that we have ignore labels.
for example in voc pascal segmentation, we have 255 for ignore areas, we have to manually set them 0 which will mess with the background.
it would be nice if we can have an option that we define the ignore labels when we use the aforementioned functions.