How to write a unit test for a video library?

I have written a video library in C++. It contains a variety of graphical widgets such as text editor, list box, button, etc.

I have also written a simple unit testing framework that suits my needs for every other library. However, I can't figure out a way to unit test a widget without requiring visual inspection and user interaction. Is it possible?

I work in an environment where 3rd party libraries aren't an option.