Provide option to dim PNG backgrounds when solo sections appear

[DerpyChap]

In the base game backgrounds, charts with solo segments usually darken the background to compensate for the increased brightness of solo sections. custom PNG backgrounds have no way of doing this, however, so they can sometimes appear a bit too bright for some users:

I think we could provide a TrombLoader option that will dim PNG backgrounds during solo sections. Video and Unity backgrounds have more control over how their background is presented, so I don't think this feature is needed there.