Add a setting to make the background a solid color instead of transparent

[quellus]

Is your feature request related to a problem? Please describe.

1. Linux support There's a lot of situations where transparency and/or Game Capture won't work on Linux machines. Lack of support for graphics drivers, certain window managers, etc.
2. Some users might just not want it to be transparent There's also some (maybe) unlikely, but valid use cases where someone just might not want the window to be transparent and would rather have a background color to chroma key.

Describe the solution you'd like There should be a setting that disables transparency and allows the user to set the background color.

Describe alternatives you've considered The only alternative I can imagine is opening a separate window with the desired background color and placing it behind GDTuber