This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

AudioOutputControl::WaitForCommand() doesn't need this check.

Prepare for adding another Cond attribute.

Since commit e81b0896, this is the
same code as in ReplacePipe().

This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a9

Make all uses of MusicChunk safe.

We can implement this now that we have a MusicBuffer reference.

This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

For remote files (not streams), this downloads as quickly as possible
to a large buffer instead of throttling the stream during playback.
Throttling can make the server impatient and it may then disconnect.

This is what Qobuz and Tidal do, and this commit attempts to solve
this by not letting the Qobuz/Tidal server wait (closes #241).

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

This is not a plugin.

Those are loaded with the "curl" input plugin, and this one is
"expensive", because it needs to send a new HTTP request with Range
header for each seek.

The absolute udisks:// URI is not usable with InputStream::Open(), and
thus we need to return the LocalStorage::MapUTF8() return value
instead.