Atmosfera

• port to 1.21.9
• fix another 2 bugs in how music was chosen - these should be the last!

the bugs resulted in custom music being less likely to play than intended and some custom tracks being skipped

• add a "Custom Music Weight Scale" option, to tune the likelihood of playing custom music

this is by default set to 250% as custom music was very rare to play, since there's so little of it