Build/acquire from source

[ProbablePrime]

We currently have a hard coded opus.dll in this repo.

Now we have our fork of OPUS available at: https://github.com/Yellow-Dog-Man/opus we need to ensure these are in sync.

Right now they aren't.

The latest version of Resonite's opus.dll works with P-Opus. The version here appears to not.

   FrooxEngine.Tests.ValueCopyTests.TestValueCopy [FAIL]
      System.EntryPointNotFoundException : Unable to find an entry point named 'opus_get_version_string' in DLL 'opus'.
      Stack Trace:
           at POpusCodec.Wrapper.opus_get_version_string()
           at POpusCodec.Wrapper.opus_get_version_string()
           at POpusCodec.Opus.get_Version()

I'm going to publish a version of this repo, with the opus.dll from the Resonite build, that appears to work. which will unblock me, but untill we resolve this issue correctly it might periodically re-appear.