Attempt to implement context sharing on Mac

[tene]

I have no idea if this is the right thing to do or not, but with this patch I can run the sharing example.

I've never done any cocoa programming at all; I just followed the "Sharing Rendering Context Resources" section of https://developer.apple.com/library/archive/documentation/GraphicsImaging/Conceptual/OpenGL-MacProgGuide/opengl_contexts/opengl_contexts.html and did what seemed obvious, and I was surprised that it actually seemed to work.

I don't have another platform to run the examples on right now to actually check if the sharing example renders identically compared to other platforms.

This will contribute to #899

[goddessfreya]

@tene Does the sharing example render the same as the window example?

You and @fschutt probably know more about this, so if it looks good to you folks, then it lgtm.

[tene]

@zegentzy Yes, the sharing example and the window example look identical to me, except for different default window sizes.