Dim settings for Screen Scale

[cdwcgt]

origin:
#17611

When using screen scale, the surrounding is not completely black

2022-08-15_22-26-41.mp4

Like this video, this is adjustable, default is 90%, but it does not plan to add to release version

Some of the information you need to know:

1. background will follow to your background source and seasonal background settings
   that means if you are a supporter, you can switch background to beatmap or skin's backgound

2. Changing him to 100% darken doesn't give a noticeable performance boost (link)
   But actually, I'm not sure because I just removed it at 100% dim settings so I think there will be some help for performance

3. The osu team once mentioned that he should be more of a switch than a slider
   if you want to make it as a switch, what dim value would you accept?

[cdwcgt]

See #23038