This adds an option to fill the ends of the generated mesh for
each spline combination with an N-gon. The resulting mesh is
manifold, so it can be used for operations like Boolean.
Differential Revision: https://developer.blender.org/D12982
I plan to use this for curve object data conversion to mesh in D12533,
and possibly for the implicit curve to mesh conversion in the curve
and text object modifier stack in the future.
Differential Revision: https://developer.blender.org/D12585
