fix aurora software audio decoding

[Antoine Martin]

Split from #845. Somewhat related to #1507.

I don't think that this ever worked properly.

Some related fixes and improvements:

• r15710 metadata buffers
• r15711 aggregate buffers before pushing them
• r15712 (+r15713 fixup) aac support

If it cannot be fixed, we should remove it.

[Antoine Martin]

• r15717 refactoring
• r15718 hande end-of-stream, skip empty buffers

[Antoine Martin]

r15732 disables all aurora codecs for trunk - only the builtin wav remains, r15733 does the same for older branches.

I've wasted enough time on this and still can't figure it out.

[Antoine Martin]

Major related issue fixed in r15783.
This fixes a number of codecs which are now parsed correctly, so r15784 removes them from the blacklist: aac and mp3 via the media source API. (and maybe this should only be done if the server exposes a flag to state that it won't send broken streams since those can crash Safari..)
Sadly, this doesn't fix software decoding via aurora. (so those codecs remain disabled there)

Related: we now handle IE sound using an http stream (r15781 + r15782)

[Antoine Martin]

Most browsers now support the audio source API, let's deprecate aurora.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1509