Add ability to change background dim color in grayscale

[santerinogelainen]

A slider in settings that changes the lightness of the background dim color. I doubt adding a full color picker is necessary

rrtyui used to play with a white/grayish bg.

[peppy]

This may require a custom shader to avoid +1x overdraw.

[Blifaloo]

It should be a custom background that can be enabled and disabled.

[Uncomfy]

I have tried implementing it with a custom shader for osu.Game.Graphics.Backgrounds.BeatmapBackground class (here's the code, it relies on updated osu-resources), however, it has downsides:

1. Doesn't work for maps with storyboards/videos
2. Forces BufferedContainer to redraw on dimming change

Does it make sense to update osu.Framework.Graphics.Rendering.RenderingExtensions.DrawQuad to support ColourOffset? That would make implementing background dim color trivial in osu.Game.Graphics.Containers.UserDimContainer class.
But I wonder if this is too big of an ask just for this feature

[peppy]

I'd probably sooner see it happening with a custom local shader.