PyAPI: re-enable the "bgl" module for headless builds

Instead of removing the `bgl` module, set all it's functions to stubs
so importing `bgl` or any of it's members doesn't raise an error.

This avoids problems for scripts that import bgl but don't call it's
functions when running in background mode.

source/blender/python/intern/bpy_interface.c

@@ -259,9 +259,7 @@ static struct _inittab bpy_internal_modules[] = {
     {"mathutils.kdtree", PyInit_mathutils_kdtree},
 #endif
     {"_bpy_path", BPyInit__bpy_path},
-#ifdef WITH_OPENGL
     {"bgl", BPyInit_bgl},
-#endif
     {"blf", BPyInit_blf},
     {"bl_math", BPyInit_bl_math},
     {"imbuf", BPyInit_imbuf},