it could be useful for audio callbacks implementations https://github.com/mfkl/libvlcsharp-samples/issues/206

Ref videolan/LibVLCSharp#591

original download link is 404 and gtk-sharp 2.x won't be receiving any update.
Will fix the CI for now.

to revert next month when new VS build is shipped to the CI agents
https://developercommunity.visualstudio.com/t/XamarinMac-binaries-are-missing-in-173/10164443

A bug was introduced in https://github.com/videolan/libvlcsharp/pull/87 which would crash the CLR
when the cleanup callback was set. The issue was that the vmem plugin does not have a Stop concept,
only closed, in which it frees up a struct containing a pointer which is used later by LVS.

So for the cleanup callback only, revert to the previous code.

Closes videolan/LibVLCSharp#339

This reverts commit 6b2665fe.

Since cc890e45, the TFM for UWP was bumped. b883bd7c missed this
sample.

they use the steps to explain how to build the repro,
instead of just sharing the built repro.