Fix Prism incorrectly rendering with dimensions=[2, 2, 2] in OpenGL

[ra1u]

Fixes Prism, that shows cube of size 2.

[behackl]

Hello, thanks for your contribution!

I have a slightly different suggestion for a more sustainable fix that takes care of the situation for all subclasses of Cube. Please take a look!

[JasonGrace2282]

Hey, I'm a little bit confused about what problem this fixes. Is there a minimal reproducible example of the problem?

[ra1u]

This bug can be observed when renderer is opengl

manim render --renderer opengl prism_bug.py

prism_bug.py:

from manim import *

class PrismBug(Scene):
    def construct(self):
        self.add(Prism(dimensions=[1,2,3]))

output:

Failed:

Expected (after patch):