Fix bug and inefficiency in request data.

The application of the mask filter was not necessary in the case of polydata input. And since the filter was specified to create one cell per point, if a polydata came in with a subset of the points as cells, that subsetting was destroyed by the filter. Also, i resulted in doubling of the memory required to represent cell points, at least in the case where the input had a single cell with all the indices as vertices.

Now the mask filter is only applied to unstructured grid input, and no cells are created in that case, as the underlying mapper handles the points just fine.