Large refactor of texture loading.

In this new implementation, everything is loaded on a separate thread,
but the textures are initialized on the main OpenGL thread.

This is all to work around the fact that you cannot create a separate
GL context in GLUT.

src/fmri/RenderingState.hpp

@@ -102,5 +102,7 @@ namespace fmri
         void renderLoadingScreen() const;
 
         void renderVisualisation(float time) const;
+
+        void loadGLItems();
     };
 }