Fixes for Mac Package

The change is needed as MSVC doesn't allow for a vector of a map
of a non-copyable object.

We can't use a list, as apparently several calls to loadAudio need
to write the blocks of different audio groups into the same position
in the output vector. std::list doesn't have random access.

Using a map now, so we can keep the previous behavior. The int64_t key
is only used in construction, and can be disregarded by the loading
code.

Audio::Samples are a move-only class, they can't be copied.

Using this non-copyable class in a std::vector<std::map<..>> does not
compile in Visual Studio 2017, see e.g.:

https://stackoverflow.com/questions/58700780/vector-of-maps-of-non-copyable-objects-fails-to-compile-in-msvc

Replacing the use of vector with list, which does compile.

MacPorts: ensure ffmpeg is installed w/ rtmpdump & non-free codecs

Link to list of 3rd party library licenses in about window

remove openal from MacPorts deps

OpenAL support was removed by VS-all  #440, replaced by PortAudio.

In a refactoring of the DELAY_LOAD stuff of Windows it seems the
Mac version was broken. On Mac there is a link to libvideostitch,
which is just a symlink to either _cuda or _opencl versions.

If the the symlink is currently pointing at the wrong one (i.e.
not the one matching our GPU), the symlink is changed and the app
is quit, user asked to launch again.

There seems to have been a refactoring done on Windows which de-
activated DELAY_LOAD completely on Mac, which made some of the
symlink changing code not compile.

Adding a new C Macro for Mac symlinks and allowing for USE_DELAY_LOAD
on Mac as well, in CMake, to restore previous behavior.