Background's design.ini should allow to over ride bg/overlay pairs

[Crystalwarrior]

Is your feature request related to a problem? Please describe.
If you have a bg folder with a lot of bgs, and you want each bg to have a simple darkness overlay, the current method is to copy-paste the same image across all the pos. This is incredibly tedious and file size inefficient.

Describe the solution you'd like
In design.ini (which we already have due to scaling support) you should be able to define bg_name = overlay_name path. The _overlay naming convention does not have to be respected.

Describe alternatives you've considered
The only viable alternative is copy pasting the same bg over and over. Witness testimony indicator is an option, but it relies on everyone having the same theme.

[oldmud0]

So

[Overlays]
bg_name = overlay_name

Okay can get behind this