sdl: audio: make AudioCVT.buf public as AudioCVT.Buf and rename AudioCVT.Buf() method to AudioCVT.BufAsSlice()
sdl: audio: make AudioCVT.buf public as AudioCVT.Buf and rename AudioCVT.Buf() method to AudioCVT.BufAsSlice()

Signed-off-by: Lilis Iskandar <3de94fd28d904a22098fd49f2d9af76f176c5fe9@protonmail.com>

veandco/go-sdl2,veandco/go-sdl2