SceneGraph: fix compilation on MinGW-w64 GCC 4.9.1.

Previously (<= 4.9.0) it behaved similarly to MSVC 2013, but the issues
with missing inline functions of classes declared as extern template
appeared again. This way doesn't work with MSVC, so I need to maintain
two separate symbol exporting scenarios. Damn you, DLL hell.
Move function pointer loading into platform-specific class.

This way it will be possible decouple the main library from
platform-specific GLX/WGL/CGL/EGL... API.
Disable TextureFormat::RGB10 for ES3.

It was defined in OES_required_internalformat which I don't want to
support in ES3+.
Platform: minor cleanup.
Platform: include windows.h in WindowlessWglApplication.

The flextGL header doesn't include it anymore so I need to include it
explicitly myself.
Using flextGL instead of glLoadGen for GL extension loading.

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.
Platform: added publicly usable *Context libraries.

They contain the same platform-specific stub as is used in *Application
libraries to load GL function pointers.
package: build GlxContext by default on Linux.