In-place rendering is implemented in such a way that there is no
change in behaviour if it is enabled or not. So there's no need
for a user setting.
Run adding mode is a LADSPA feature that plugins can support. Using
that mode, they don't replace an output buffer, but add their rendered
audio to it (with a user definable gain). A very necessary feature to
be able to use a LADSPA reverb in place of the internal reverb.
Removes the need for VLAs before ladspa call in rvoice mixer and number
for for loops to set up and advance the buffers. Also reduces the number
of state changes (and atomic operations) by calling fluid_ladspa_run only
once.
