Loading of OpenGL extensions for each context.

[maartenuni]

Currently a we create windows by using SDL2. When the window is created a opengl context is created. When we create the context we can start to load OpenGL extensions. We should do this for every context. Additionally, we should take care that if a context is created it supports all of the required features. If not all of the required features are supported, the creation of the window should be terminated, since it isn't functional window.

[maartenuni]

Also when a context is made current, we should make sure the function pointers of that context are active and not the pointers of an other context.
Information about how to do this is found at the website of lib libSDL2 https://wiki.libsdl.org/SDL_GL_GetProcAddress