It's impossible to do graphics in pure C, so what are the alternatives? Whatever you do at whatever level of abstraction/portability (SDL, OpenGL, Metal, Vulkan, Unity, ...), you're leveraging zillions of lines of other people's code.
The only way arguably you could do it in pure C (with undefined behavior) is in enviroments that let you write straight to graphics framebuffers without any abstraction layer and even when you can't do keyboard or mouse input in C without library and/or kernel support.
The only way arguably you could do it in pure C (with undefined behavior) is in enviroments that let you write straight to graphics framebuffers without any abstraction layer and even when you can't do keyboard or mouse input in C without library and/or kernel support.