Eliminate a use of GetGlobalConfig().

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

Fixes valgrind warning.

Fixes another part of #208, the one which caused the bogus exception.

Closes #208.

Wired to Filter::Flush().

Closes #153

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

Wohooooo, the method Filter::Reset() has been broken because no
implementation of it has ever been called for a loooong time.
And nobody ever noticed it.  WTF.

Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

These are not in filter_plugins[] and have dedicated constructors
anyway.

Circumvent filter_new() with a fake ConfigBlock.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Not used.  Force implementations to initialize out_audio_format.