forward port lv2_evbuf update (from jalv)
fix processor_lock

Only WriterLock requires to hold process_lock() as well
otherwise Route::process_output_buffers() may deadlock
fix stem-export buffer-size check
remove cruft - unused _size in audio-buffers
update audio-buffer assert, take offset into account
don't display debug message in no-debug builds
VBAP nomenclature s/Direction/Azimuth/
fix typo ndef, NDEBUG
merge (w/fix) with master