Extend platform support for WSI without the need to include windowing API

[RedNicStone]

At the moment volk allows the user to use WSI functions for the Win32 platform without having to include windows.h. This is a very useful feature not only to prevent unwanted definitions and reduce compile time but it also allows the developer to separate code handling windowing from the code that handles rendering.

Unfortunately this is not currently available on any other platform. I have identified the platforms that could benefit from this feature and added it on my fork. In addition to that I added guards that prevent redefinition of types in case the user includes the library (not that they must be included before volk.h for this to work).

I think this is a good change to take into the master branch.