LV2 - use scratch buffers to scratch :)
catch assert()
initialize some uninitialized values
fix typo in c212c16eb
initialize two more uninitialized vars..
valgrind ignore glib UTF8 warnings
fix close_allv() sentinel
correct error message
clean up some valgrid "Uninitialised value was created by a heap allocation" wanings
tiny detail.. GUI-callback may try to check vmonitor->is_started() or issue a periodic seek update while it's going away
use zeroed scratch buffers for "silent" plugin runs

Plugins rewrite the buffer data in-place and some plugins
can produce output even when fed with silence.

Hence, during a PluginInsert::silence() run a plugin can
inject data into the "silent" buffers which causes side-effects.

Kudos to Chris 'oofus' Goddard for finding this issue.
re-check meterbridge metric-area on route deletion

fixes http://tracker.ardour.org/view.php?id=5616#c15204
add _ISOC9X_SOURCE define to compiler command line to provide uniform access to llabs()
remove odd namespacing typo
remove an identical odd namespacing typo
rework MIDI [processor|plugin] chain

* forward midi-data around plugins that have no MIDI-out
* allow to insert plugins with no MIDI-input at a point with one MIDI-channel

This works because excess ports (both plugin and route) remain
unconnected and use scratch-buffers.

Tested with LV2, LXVST and LADSPA.
(AU plugins with variable in/out retain the old behavior, no bypass)

fixes http://tracker.ardour.org/view.php?id=5630