prevent more than one decoder when loading SoundFile from stream

[rpvelloso]

When a soundfile is loaded from a stream, chunk by chunk, it can't be played simultaneously by multiple sound objects (i.e., it can't be shared) because the stream pointer can't be shared (it could be shared using mutex and alike, but we want to avoid this). So a stream soundfile should be allowed to instantiate only one decoder. If sharing is needed then the soundfile should be loaded to memory so it can be played simultaneously by several sound objects (or, if the file is too big, multiples soundfile should be created for the same disk file).