A perfect implementation of this would probably leave the filter/header
content visible, but that requires some re-thinking and restructuring to
how the content is displayed in these overlays (ie. the header
component shouldn't be inside the `ScrollContainer` as it is fixed).
Supersedes and closes#10774.
Closes#933.
Addresses most pieces of #7417.
In theory this seemed like a good idea (and an optimisation in some
cases, due to lower fill rate), but in practice this leads to weird edge
cases.
This aims to do away with the operations on external drawables by
applying a dim to the area behind the `LoadingLayer` when required.
I went over each usage and ensured they look as good or better than
previously.
The specific bad usage here was the restoration of the colour on dispose
(if the `LoadingLayer` was disposed in a still-visible state).
I'm aware that the `BeatmapListingOverlay` will now dim completely during
load. I think this is fine for the time being.
