Remove direct access to `neoscore.background_brush`

[ajyoon]

neoscore.background_brush is a global variable that users are expected to set using neoscore.set_background_brush, but this creates a very easy error for users understandably attempting to change the background brush by simply saying neoscore.background_brush = Brush(...).

We should update this to make the global variable hidden by naming it neoscore._background_brush, and then making access to it go through a new function neoscore.get_background_brush() -> Brush.

This is a breaking change, but I think worthwhile. The background brush is very rarely read by users anyway.