C Graphics

Anyone know a good place to learn how to code OpenGL and GTK (or something similar)? I'm trying to do this in pure C, and stay cross-platform, which kind of rules out things like DX, WXwidgets, and Win32. I'm looking for a website or book or something, not a course.

I'm OK if i need to work with a little C++, but want to keep my code as C as possible, for reasons of performance, style, preference, and usage.