Implement cubemap generate operator

This operator takes a file as an argument and builds a cubemap from it.
Valid options are to supply the output from Plasma's
Graphics.Renderer.GrabCubeMap console command. The operator will find
the other five files and generate a cubemap with the faces saved by
Plasma. Otherwise, any arbitrary image can be supplied. If the filenames
do not fit the expected format, any missing faces will be replaced by
the face specified in the file selector. This will generally result in a
cubemap with six identical faces.

korlib/korlib.h

@@ -29,6 +29,7 @@ class PyObjectRef {
     PyObject* m_object;
 
 public:
+    PyObjectRef() : m_object() { }
     PyObjectRef(PyObject* o) : m_object(o) { }
     ~PyObjectRef() { Py_XDECREF(m_object); }