On desktop this saves about 50 kB in symbols. Was done for Vulkan
already, this follows that (two years later). I need this in order to
solve the problem of static globals being unique across shared libs, and
it sounded better to export just one symbol instead of 689.
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.