Introduce ladspa::Adapter and GVerb unit

[markandrus]

This PR introduces a mod for LADSPA units. Build it like this:

make ladspa

There is an adapter, ladspa::Adapter, and a sample GVerb unit copied from Steve Harris' LADSPA plugins. It would be interesting to port the remainder of Steve Harris' plugins and the CMT Library Plugins. Are there any other good LADSPA plugin sources?

Also, I believe the LADSPA_Descriptor contains enough information to generate the GVerb.cpp and GVerbUnit.lua. I don't know what this would look like yet…

Warning

I think this approach may have licensing problems. The LADSPA header is LGPL, which AFAIK can be statically linked in er-301 builds; but GVerb and the adapter are GPL, which AFAIK cannot. If these were built and loaded separately, I think it would not be a problem.

Note

I started working on a version of this that will dynamically load pre-compiled LADSPA plugins (dlopen). That should resolve the licensing problems, and also improve maintainability.

Building for and testing on the hardware

I built it for the hardware by running make in Docker (thanks, @tmfset, for the image):

docker run -it --mount type=bind,source=$(pwd),target=/mnt tomjfiset/er-301-am335x-build-env:1.1.2 /bin/sh -c "cd /mnt && bash"

After testing it, I see the GVerb unit takes approximately 15% of CPU (the CPU was at 5% before I loaded the GVerb unit):

er-301-ladspa-gverb.mov