PlayBGM and Autotile fixes

BGM does not restart anymore if PlayBGM plays the same BGM again (this is used to adjust Volume and Speed). Fixes low FPS in Hero's Realm on the worldmap.

Can anybody check if this is the correct way to do it with OpenAL? Can't test this backend.

Also fixes problems with transparent autotiles, now parallax background renders correctly in Yume Nikki:

http://i.imagebanana.com/img/8timzemw/yumme.png

(yes this is completly unrelated with BGM but was a one line fix...)