LibVLC loading refactoring for macOS

Problem to solve

To be able to use libvlc builds from official releases, we need to be able to load and P/Invoke dynamic libvlc macOS builds.

Currently, it's all statically linked together in a libvlc.dylib binary. The libvlc builds from vlc.dmg are organized similarly to the libvlc windows ones (plugins folder, lib folder, share folder).

Intended users

macOS users.

Proposal

There are a few challenges...

• We need to perform this refactoring while not breaking existing platforms (netcore and mono).
• We need to perform this refactoring while keeping the current platform versions number supported (i.e. any netcore version).

mono/cocoa support is fine, issue is with netcore.

Problems are two fold:

• To be able to re-use the current P/Invoke code, we need to keep the dllimport statements as they are currently. In theory, one could load manually the dylib, and all further dllimport calls would look at the currently loaded dynamic librairies before trying a new dlopen to perform the dlsym. That's the theory anyway https://github.com/libgit2/libgit2sharp/blob/7fc4be5193dbdd08538b4b150332b5a73770e0f6/LibGit2Sharp/Core/NativeMethods.cs#L40

However, that's not what I observe. And for the netstandard/netcore build, that means the dlopen call argument is different on mac and windows (@rpath/libvlc/libvlc) and it should not.

• Even when using the non standard @rpath/libvlc dlopen argument and setting the correct VLC_PLUGIN_PATH, libvlc_new returns null.

https://github.com/mfkl/libvlcsharp/commits/mac-libvlc-loading

https://github.com/mfkl/libvlc-nuget/blob/mac-loading-rework/build/VideoLAN.LibVLC.Mac.targets

Documentation

Just the cherrypicking enabling whenever this works.