audio: move ready-for-ao data buffer from decoder to AO

audio: move ready-for-ao data buffer from decoder to AO

Move the buffer storing audio data ready to be fed to the audio output
driver from the audio decoder object to the AO object. This will help
encoding code deal with end of input, and may also be useful to
improve other general gapless audio behavior (as AOs which do not
accept chunks smaller than a certain size may keep them in the buffer
while the decoder changes).

Less data may be dropped now when changing audio filters or switching
timeline parts.