Loading Open GL

While the usage of Open GL is basically the same across platforms, the precise details of how to first initialize Open GL varies.

The general idea is that we need to do two things:

1. Create an Open GL context and make it "current". A GL context is what holds all of the drawing state for GL. Each GL Context must only be current in a single thread at a time, otherwise you'll get undefined behavior. Also, a thread's current context is a thread-local variable, so you can't have more than one context current in a thread at a time. Also, a context is associated with a particular window's drawing area. Usually you have just one window, and so you have only one GL context, meaning that don't need to worry about any of that. If you're trying to use GL with more than one window at a time, things can get tricky.
2. Load the GL function pointers. Unfortunately, you can't access GL like it was a normal dynamic library. At least, you can't on Windows and Linux. We'll have a whole fun time loading function pointers manually.

Acknowledgements

For this portion of the guide I'd like to give an incredible acknowledgement to glowcoil, who wrote the raw-gl-context crate, which gives come very clean and clear examples of how you open a GL context on the major platforms.