Add background color support and fit mode

[anufrievroman]

Hi, I am a maintainer of waypaper app, that uses wallutils as one of the backends for wallpaper setting. I have a couple of questions:

1. While all other backends support background color (color behind the wallpaper), I can't find the option to set it here, sorry if I missed. Do you plan to support it?

2. Classically, wallpaper mods are usually either fill, fit, center, stretch, or tile, which are all different things. However, setwallpaper --mode only supports center, stretch, scale and tile. So, what's exactly the scale mode, and could you please add one more mode to be more uniform with other wallpaper tools?

Thank you.