Fixed typo in the sfml-graphics CMake file
Clarified Window::setIcon() ownership semantics
Replaced GLEW with loader generated by glLoadGen, restructured GLExtensions.hpp for easier extension bookkeeping, make use of GLEXT definitions in Shader.cpp and Texture.cpp as well.
Cleaned up GL extension handling a bit more, replaced GL_MAX_TEXTURE_COORDS with GL_MAX_TEXTURE_IMAGE_UNITS, added extension support checks to the constructors of GL objects to warn users earlier of features their platform doesn't support, replaced manual wgl extension loading by glLoadGen as well, removed GLLoader.cpp and GLLoader.hpp from projects on OpenGL ES platforms.
Made use of glLoadGen for GLX extension loading as well, implemented flags for requesting a debug or core/compatibility profile context.
Removed check from Shader constructor, removed check from ContextSettings constructor, added warnings to all context creation implementations to warn the user of an incompatible mismatch between the context they requested and the context that was created, changed the default context version from 2.0 to 2.1.