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.
