BUG: fix issue with the mask points filter.

With VTK-5.10 (for sure), the filter tries to allocate the default
maximum number of points which is impossibly large.  To work around this
we explicitly set the max number of points to those in the input.
Nudge the test script to flush the pipeline.

This is a hack but it is strange that it works for every other use case
and many other tests.
Merge pull request #106 from prabhuramachandran/fix_mask_points

Merged.