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/Drawable.cpp

@@ -14,3 +14,8 @@ void fmri::Drawable::patchTransparancy()
         (*it)[3] = alpha;
     }
 }
+
+void fmri::Drawable::glLoad()
+{
+    // Do nothing
+}