Mobject.split() behaves differently between Cairo and OpenGL

Description of bug / unexpected behavior

As mentioned in #4914, the behavior of split is different between the two types of Mobject. Here is the implementation of Mobject.split:

def split(self) -> list[Mobject]:
    result: list[Mobject] = [self] if len(self.points) > 0 else []
    return result + self.submobjects

and here is the implementation of OpenGLMobject.split:

def split(self) -> Sequence[OpenGLMobject]:
    return self.submobjects

In short, the Cairo version potentially includes the calling mobject itself if it has any points, and otherwise simply returns a copy of self.submobjects. The OpenGL version never includes the mobject itself and it doesn't copy self.submobjects but simply returns the list itself.

It seems like this difference has existed since OpenGLMobject was introduced in the library, and nothing has broken horribly, so probably this isn't a huge issue in Manim's own source code. It might confuse users who switch from one to the other, though.

Expected behavior

I expect both types of Mobject to behave identically. I don't know which is the correct behavior; the method has no docstring and I haven't been able to find a case where split is called on a mobject which both has 1) points and 2) submobjects, so it isn't clear to me how to find out.

How to reproduce the issue

class SplitTest(Scene):
    def construct(self):
        mob = Circle().move_to((-2, 1, 0))
        mob.add(Square().move_to((0, 1, 0)))
        mob.add(Triangle().move_to((2, 1, 0)))
        self.add(mob)
        self.add(mob[:].copy().shift(DOWN * 2))

Additional media files

This is the output when rendering with Cairo:

This is the output when rendering with OpenGL:

Logs

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System specifications

• OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
• RAM:
• Python version (python/py/python3 --version):
• Installed modules (provide output from pip list):

PASTE HERE

• LaTeX distribution (e.g. TeX Live 2020):
• Installed LaTeX packages:

Additional comments