Don't glyph masked points in vtkUniformGrid.

vtkGlyph3D would glyph masked points in a vtkUniformGrid. Adding logic
to avoid that. (In the past, this was handled in ParaView's subclass of
this filter).

Also making vtkGlyph3D easier to subclass for handling cases like iterating
over composite datasets.

Change-Id: I08f2fa7ff79a249130753f0dacc4187f51944584
Merge topic '14279_glyph_filter' into master

82a0c473 Don't glyph masked points in vtkUniformGrid.