Is the input `mask` modified and returned?
Or is the returned mask a new mask or copy of the input mask and only the shape of the input mask is relevant?

Shouldn't the `mask` have a world coordinate system attached?
Or is the region defined in pixel coordinates?

I think this is the equivalent of the `region.get_mask` method in [pyregion](http://leejjoon.github.io/pyregion/users/overview.html#use-regions-for-spatial-filtering)?
```
mask = region.get_mask(shape=(300,300))
```
Since it's part of the API, +1 to a more explicit name for this method.