Tool for annotation of images for semantic segmentation

I have been searching around for a software tool, that I can use for annotating images. More specifically I want to do annotation to be used for semantic segmentation, meaning I want to create masks. I want to be able to create training data for applying a segmentation CNN (like for instance U-net).

However I have been digging around the internet, and I have tried out some options. But I have not really found anything that seems to do the trick.

I do not expect to have many classes, but I probably want to be able to be able to annotate more than two classes (something, background). And of course every pixel has to belong to a class.

So what I am looking for is a relatively simple tool that either lets you mark areas of a class with a brush or that lets one mark polygons and then fill those, and then the outside the polygons as background. So can anyone point me to anything like this? And share their experience?