No output at all is fine.  If the partition needs an output, it must
be moved there, but having just a null output isn't helpful.

By eliminating GetAnyClient(), we can have instances with no outputs
at all.

Make idle events per-partition, but leave Instance::EmitIdle() and its
underlying idle_monitor which broadcasts idle events to all
partitions.

For efficient traversal within one partition, e.g. for distributing
partition-local idle events.

These are global events.

One basic library without dependencies, and one with heavy
dependencies like libsamplerate.

Prevents compiling those sources many times, once for each debug
program using it.

There's just one caller, i.e. ScanGenericTags(), which is documented
to throw exceptions.

This reverts commit c84bae73.  A
configuration option is not necessary, because the PcmConvert
constructor knows already whether integer or floating point is needed.

release v0.21.19

Google Play refuses uploads with a lower targetSdkVersion.