Using lock() to enforce external synchronization

[Mervill]

Vulkan spec §2.5. Threading Behavior:

All commands support being called concurrently from multiple threads, but certain parameters, 
or components of parameters are defined to be externally synchronized. This means that the 
caller must guarantee that no more than one thread is using such a parameter at a given time.

There is currently an experimental feature of the generator, LockExternSync which when set to True will generate a lock() statement in commands that use arguments marked as ExternSync and are not also optional.

In general, I don't have as much experience in multithreading as I'd like, and threading in C# can get pretty muddy when things like async are brought into the picture.

Ideas about how to make Tanagra work with C#'s threading infrastructure would be greatly appreciated.