When in doubt, it is usually better to prefer composition over inheritance. Still, that is just a heuristic, not a law, and asking if this heuristic applies to your specific case is definitely justified.
Here is a simple argument why I think you should prefer composition in this case: if you want to keep your design open to make Drawable a base class of certain more specific "drawables" inside your library (like 2D or 3D drawables, or OpenGL vs. DirectX drawables, or some other kind of technical classification), use composition. Such a technical inheritance hierarchy will not interact well when mixing it with a domain specific inheritance hierarchy outside your library, where ProgramObject may be the base class of certain domain specific class (like a player character, or a vehicle, or some kind of other object).
This article here calls these aspects different dimensions:
When you have a situation where either composition or inheritance will work, consider splitting the design discussion in two:
- The representation/implementation of your domain concepts is one dimension
- The semantics of your domain concepts and their relationship to one another is a second dimension
In general, inheriting within one of these dimensions is fine. The problem becomes when we forget to separate the two dimensions, and start inheriting across inter-dimensional boundaries.
And this is exactly what would happen when you start to inherit a ProgramObject from a Drawable: you will mix up the "drawing" dimension with other domain concepts, and that is often a way to desaster.