This filter should sample a MaskGrid and only accept points where sampling returns true.

An example constructor (based on BBoxFilter):

MaskFilter(const openvdb::math::Transform& pointsTransform,
    const openvdb::MaskGrid& mask);

This needs to account for and be efficient in situations when the transform of the MaskGrid matches the points transform and cases where they don't. I suspect that it may prove more efficient to resample the mask grid into an index-space mask grid where they don't match and perform the sampling there (similar to BBoxFilter) but we'll need to benchmark this. I'm marking this task as related to fast mask sampling as a result - OVDB-21.