sdl: Make AudioSpec a struct, then fix Audio.open(), load() and mix()
sdl: Make AudioSpec a struct, then fix Audio.open(), load() and mix()

Fixes bug 669279.

rvu95/vala,severinh/vala,frida/vala,Distrotech/vala,playya/vala,tjol/vala-tjol,GNOME/vala,tjol/vala-tjol,jessevdk/vala,GNOME/vala,xdave/pvala,jessevdk/vala,flobrosch/vala-fixes,Distrotech/vala,frida/vala,frida/vala,flobrosch/vala-fixes,flobrosch/vala-fixes,frida/vala,spidermonkdust/vala,Distrotech/vala,GNOME/vala,rvu95/vala,tjol/vala-tjol,spidermonkdust/vala,jessevdk/vala,frida/vala,rvu95/vala,severinh/vala,playya/vala,severinh/vala,playya/vala,spidermonkdust/vala,GNOME/vala