CGL
CGL (C Game Library) is a multipurpose library
...Designed for simplicity and portability, cgl allows rendering of primitives such as lines, circles, triangles, and text to an in-memory framebuffer, which can then be displayed with any platform-dependent backend. It’s ideal for building custom engines, retro-style games, GUIs, or educational demos where GPU acceleration is not required. Its small footprint and lack of external dependencies make it easy to embed in any C project.