Custom BGM is fixed
Based on this commit https://github.com/hrydgard/ppsspp/pull/5828 on improvements to sceMp3
Custom BGM can be update now. Test on "Miku" is just all right :)
Merge pull request #5829 from kaienfr/new_Custom_BGM

Custom BGM is fixed
specify number of channels
change old comments
Merge remote-tracking branch 'origin/master' into master_0.9.8
Merge branch 'sceMp3_fix' into master_0.9.8