OpenGL context without an SDL_Window.

[icculus]

As seen in #12678 specifically, it might be nice to be able to create a GL context without an SDL_Window (SDL_GL_CreateContext(NULL)). This is a task to investigate if this is feasible and if so, implement it across backends.

[mariob92]

Something like pBuffers?

[icculus]

Sort of, yes.

[icculus]

Just committing to investigating this for 3.6.0. We might still abandon this, though.