Add ability to change background dim colour in grayscale

[Uncomfy]

Previously discussed here: #12983

Depends on:

• Add a custom shader for BeatmapBackground to handle background dim colour changes osu-resources#344
• Expose spites and drawnodes in BufferedContainer, TextureAnimation and Video to make it easier to inherit and tinker with osu-framework#6395
• Allow to do DrawNode invalidation on cached BufferedContainer without redrawing the framebuffer osu-framework#6404
• Split getRoundedColor into two separate parts osu-framework#6591

Allows to change which colour the background is dimmed to. Currently only supports grayscale.

This is not a full implementation, as it doesn't support storyboards and videos.

The main reason I'm asking for comments so early is that this implementation handles dimming in two different classes, which leads to code duplication, so I wonder if there is a cleaner way to do this.

osu_bg_dim_colour.mp4

Here are also a few examples of how it could look with colour picker instead of a grayscale slider, using SettingsColour, but I'm unsure if it fits the design, as colour pickers weren't used in Settings/Visual Settings before. Also it doesn't work properly in Visual Settings yet, because it doesn't have a PopoverContainer in its hierarchy, and I'm not really sure how to handle it.

[Uncomfy]

Storyboards are now also supported

storyboard.mp4

However, I used default method implementation in IColouredDimmable interface, unsure if that's ok to do

[Uncomfy]

Current implementation makes performance on heavy storyboards (such as Mili - world.execute(me);) significantly worse. Mean frametimes for different parts of the map:

	Gameplay	Breaks	Fade in	Fade out
2025.607.0	3.51 ms	3.56 ms	3.43 ms	3.68 ms
80c93b3	4.32 ms	4.34 ms	4.22 ms	4.50 ms
Relative increase	23%	22%	23%	22%