Transparent background impossible with OpenGL renderer

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## Description of bug / unexpected behavior
When you try to export a video with the `--transparent` flag and the `opengl` renderer, the output video doesn't have any alpha channel.

## Expected behavior
Rendered video should have transparency (alpha channel).

## How to reproduce the issue
Use a `ThreeDScene` or `Scene` (the problem is the renderer, not the scene) with some `OpenGLRectangle` (or `OpenGLMobject` you want), set the `self.camera.background_opacity = 0` and use the `--renderer opengl` and the `-t` tags/flags.

## Additional comments
If you check the `opengl_renderer.py` file, that is the specific renderer that takes part when you send that flag, you can see in the line 596 that the background color is forced to have `1.0` as alpha background. This means that the background will never be transparent. That alpha value should check if the flag `--transparent` is set as `True`, as the `cairo_renderer.py` does, or at least allow you changing that value.

Even in the `update_frame` method of that file, if you manually force the `background_color` to be (0, 0, 0, 0) as a numpy array, it is set as `1.0` for the alpha as the setter is forcing it.

Start in opengl_renderer.py at the background-color handling around line 596 and inspect update_frame, then compare the transparent-background behavior in cairo_renderer.py. Trace how the --transparent or -t flag reaches the OpenGL renderer and verify that exporting a Scene or ThreeDScene with background_opacity set to 0 preserves an alpha channel.

Written by the indexing model from the issue text.