Made tests for surrounding rectangle and stream lines pass

[henrikmidtiby]

I have identified some tests that I would like to fix in experimental. The tests can be evaluatated by this command.

pytest tests/module/mobject/types/vectorized_mobject/test_stroke.py

Now I have tried to make the tests pass, by changes like

• changing base class from VMobject to OpenGLVMobject
• fill_opacity -> get_fill_opacity()

Is this a suitable approach?

Further Information and Comments

To make the commit I had to ignore all mypy errors. Is that the preferred approach?

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