GL: clean up the includes.

No need to include all GL headers just for one thing.

src/Magnum/GL/GL.h

@@ -34,6 +34,7 @@
 #ifndef DOXYGEN_GENERATING_OUTPUT
 typedef int GLint; /* Needed for sampler enums */
 typedef unsigned int GLenum; /* Needed for *Format and *Type enums */
+typedef unsigned int GLuint; /* Needed by Implementation/State.h */
 #endif
 
 namespace Magnum { namespace GL {

src/Magnum/GL/Implementation/State.h

@@ -27,8 +27,8 @@
 
 #include <memory>
 
+#include "Magnum/Magnum.h"
 #include "Magnum/GL/GL.h"
-#include "Magnum/GL/OpenGL.h"
 
 namespace Magnum { namespace GL { namespace Implementation {