Do not litter client code with #ifdef WITH_OPENGL.

The Renderable class now encapsulates construction and destruction of
graphics resources and computation of texture coordinates.  This
minimizes the littering of client code with '#ifdef WITH_OPENGL' and 'if
(OPENGL)' clauses. In fact there should never be any of these from now
on.
Adapt to changes in Renderable interface.

Adapt the classes already capable of OpenGL rendering to the new
Renderable interface for managing resources. Their code is now copletely
transparent to the rendering device in use.