Improved setting of OpenGL colors

[k4pran]

Overview: What does this pull request change?

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

Fixes #2087

In migrating tests from cairo to opengl some tests were failing due to fill_color, stroke_color and their respective opacities not being set. The background_color was also always being set from config, so I made it a property to make it easier to change without changing the config.

class Test(Scene):
    def construct(self):
        self.wait()
        self.renderer.background_color = YELLOW
        self.wait()
        self.renderer.background_color = GREEN
        self.wait()

After these changes I could run the tests for color with opengl and added some new ones for fill and stroke

test.mp4

This also makes it more consistent with cairo with the exception of background-opacity. I couldn't set background opacity for opengl, and from what I can tell it isn't trivial to do.

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]

Looks good, and thanks for adding these nice tests as well!

There is just a minor change request that I have, I've left a comment -- but otherwise I think this can be merged.