Bugfixes & Music commands

Fixed: INIGET$ returns "NO_DATA" if data does not exist
Fixed: NETWEBEND redirects properly
Fixed: ALPHAMODE behaves more like GLB (but not quite the same)
Fixed: PRINT with unknown characters does not crash - ignores them
Fixed: SMOOTHSHADING works properly
Fixed: MEM2SPRITE works again on newer browsers
Fixed: SOX with libmad
Fixed: MUSIC playback & looping
Added: MUSICVOLUME
Added: PAUSEMUSIC
Fixed: ALPHAMODE 0.5; SHOWSCREEN did not cause weird graphical gliches