Rework AVFRingBuffer to return -EAGAIN when there is no data

[danielk]

Traditionally we've always had RingBuffer::Read() return the number of bytes requested unless we are at the end of the file. But this blocks libav which expects this the read to complete fairly quickly even if it is a blocking read. Instead we should block for only a short time waiting for more data and then return what data we do have. To avoid premature exit from the decoder loop we also need to add EAGAIN returns so that if we don't have more data right now, but expect to have some in the near future we can signal this to libav.

[danielk]

Initial patch