Make Vulkan Pfn* structs disposable to avoid memory leaks

[Perksey]

BuildTools-generated Pfn* structures implement IDisposable so that SilkMarshal.Free can be called to unpin the delegate and allow the GC to have at it. The Vulkan Pfn* structures, which are manually written, do not implement this at the moment and cause (very minor) memory leaks.

The work is:

• look at how the Pfn* structures in one of the other libraries (for example SDL) does it
• make the Pfn* structures in Vulkan inherit from IDisposable
• implement it
• optionally check OpenXR (and maybe OpenGL) as well

Community contributions welcome.

[p3t3rix]

#615
Currently only prevented by the CI-build.

[Perksey]

Fixed by #615