Update opengl_vectorized_mobject.py

[yang-tsao]

The "insert_n_curves_to_point_list" function requires the "points" argument to be a numpy array, since it calls the "get_bezier_tuples_from_points" function, which requires "points" to be a numpy array because it has the "return points.reshape((-1, nppc, 3))" statement. Ordinary lists do not have a "reshape" method.

So we need to convert "sp1" and "sp2" to numpy arrays before calling the "insert_n_curves_to_point_list" function.

Overview: What does this pull request change?

Motivation and Explanation: Why and how do your changes improve the library?

I fixed the get_nth_subpath function, so it will work with the opengl renderer.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested