When allowing to cross-connect plugin-ports, inplace processing can
no longer be used. We need a complete set of independent input and
output buffers.
Since scratch and silent buffers are used by the various plugin
implementations we cannot re-use them in the PluginInsert.
Besides we need a complete BufferSet which can hold both: ins + outs.
This is needed for gain and pan automation buffers
as well as silent and scratch buffers when bouncing or
exporting with larger chunk size than the current engine
period.
"scratch buffers are by definition scratch and their contents are undefined at all times"
"silent buffers are by definition all-zero and should not be used for real data"
But track & route were using those for actual data; plugins (which may run
in the same thread and may get the same buffers) use them for scratch thereby
overwriting real data.
In particular get_silent_buffers() (used by LadspaPlugin::connect_and_run)
clears the buffer which can holds real data:
e.g. via Route::passthru_silence() -> plugin1 -> plugin2 (clears output of plugin1)
