Change SDL_VIDEO_GL_DRIVER and SDL_VIDEO_EGL_DRIVER environment variables to hints?

[slouken]

This is the last remaining item from https://github.com/libsdl-org/SDL/issues/7702

The SDL_VIDEO_GL_DRIVER and SDL_VIDEO_EGL_DRIVER environment variables are used by EGL, and I wasn't sure how to explain clearly why you'd set an OpenGL library that is used by EGL, and why this hint would be different from SDL_HINT_OPENGL_LIBRARY.

We should probably convert these to hints and clearly explain why they exist and when you'd want to use them, or just punt and leave them as environment variables.