this is required for mods such as flashlight which always want to draw
objects above hitcircles. If just adding children to a RulesetContainer
the hit circles would always be above the added children (no matter
the Depth).

There are now skinnable actual blinds (shoji screen panels)
The black overlay is still behind them to avoid cheating with skins
The blinds don't open linearly anymore, they are health squared now
When easy mod is on, there is always a little gap open

Changes how OsuModBlinds passes the playfield and how DrawableOsuBlinds
handles the restrictTo parameter.

Not sure if we want this going forward.

We decided that mods shouldn't be interacting with other mods. This can be added once we have the ability to have per-mod settings, as a difficulty setting local to blinds.