Fix: OpenGL BackgroundRectangle infinite loop

[camburd2]

Overview: What does this pull request change?

Fixes #3634 by modifying adding a line and changing a line in opengl_vectorized_mobject.py. After reproducing the issue with the given test case, I found that there was an infinite loop involving calls to get_color() in opengl_vectorized_mobject.py for the background object.

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

I compared opengl_vectorized_mobject.py (OVM) to vectorized_mobject.py (VM) used by cairo. I noticed the if condition was the only difference. The condition in VM did not check stroke but instead checked fill opacities.

I essentially made the OVM condition equivalent to the VM condition - using OVM's has_fill(). This change, along with adding a missing attribute (mobject_style["fill_opacity"]), fixed the bug. For reference:

in vectorized_mobject.py:

    def get_color(self) -> ManimColor:
        if np.all(self.get_fill_opacities() == 0):
            return self.get_stroke_color()
        return self.get_fill_color()

current opengl_vectorized_mobject.py:

    def get_color(self):
        if self.has_stroke():
            return self.get_stroke_color()
        return self.get_fill_color()

    def has_fill(self):
        fill_opacities = self.get_fill_opacities()
        return fill_opacities is not None and any(fill_opacities)

proposed opengl_vectorized_mobject.py:

    def get_color(self):
        if not self.has_fill():
            return self.get_stroke_color()
        return self.get_fill_color()

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

[behackl]

This seems fine to me, thanks! It would probably be nice to include a non-graphical test in tests/opengl/test_opengl_vectorized_mobject.py -- would you be willing to look into this, @camburd2?