Make OSD progress bar's completed section color configurable or solid white

On a white background, the default progress bar basically consists of a single black line separating watched from unwatched.

That doesn't really stand out as much as one might hope. It would be nice to have the completed and uncompleted sections be more distinctly colored.

The uncompleted section doesn't actually have a color, so I used to use osd-back-color="#77000000" to give it a gray color, which stood out nicely against the solid white of the completed section.

But sometime in the past year the progress bar's completed section started using what appears to be a non-transparent version of the back-color and now the difference between completed and uncompleted can be impossible to see on a white background.

I recommend:

• User can set a color for the completed section of the progress bar.
• If no color is set, use solid white not the back-color.

Start at the OSD progress bar implementation and the osd-back-color option. Trace how the completed and uncompleted sections currently derive their colors, then assess the requested configurable completed color or solid-white fallback. Done means the two sections remain visibly distinct on a white background and the existing option continues to work.