Update VideoClip.py

[aaa3334]

Adding in a new optional parameter bg_radius. This parameter allows users to round the edges of the background text box so it is not sharp cornered.

This new parameter is optional and utilises the pil libraries rounded_rectangle.

https://pillow.readthedocs.io/en/stable/reference/ImageDraw.html#PIL.ImageDraw.ImageDraw.rounded_rectangle

This has been around since pil 8.2.
Moviepy requires: "pillow>=9.2.0,<11.0",
Therefore, this is compatible.

Note I have extensively tested on my local computer. I decided to make the change as it was much simpler than me manually creating this on my end (I have also seen others eg. on reddit with the same issue).

I have not done many PR's before - please let me know if I have not done this correctly. I tried to follow the documentation but as I am newer to this with git I am unsure if I did it in the right place etc.

• [ x] I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/ (I did this on my local computer but am unsure how to add it for this PR)
• [ x] I have properly documented new or changed features in the documentation or in the docstrings
• [ x] I have properly explained unusual or unexpected code in the comments around it