Version query. Such a simple and obvious thing and because they forgot
to add such function to Vulkan 1.0, the only three possible ways to
retrieve a version retrieval function are:
1. with a static constructor that could make a library crash even before
main() is reached if the driver is *extra* shitty
2. with a init() function that would cause race conditions if ever
called from multiple threads (which of course can happen because you
need to know instance version in order to use the correct function
pointers and route pNext fields)
3. with a init() function that internally uses std::mutexes and
std::call_once and <functional> and atomics and whatnot and thus
takes longer to compile than the rest of the engine
Approach 2 chosen originally but the race condition countermeasures
turned out to be extra annoying to use, so switched to approach 1 now.
The function pointer loading is now moved to Application classes to make
it possible to decide about platform-specific API at usage time, not at
library compilation time.
Currently it's not possible to create the Magnum context any other way
than through Application classes, will solve that in next commits.
Vladimír Vondruš