MacOS - set compatability flag for GL3

[NaLiJa]

Fix GLFW error (0x10007): NSGL: The targeted version of macOS only supports forward-compatible core profile contexts for OpenGL 3.2 and above

[mikke89]

Thanks for the pull request! I looked into the deprecated features, and we're definitely not relying on any of those. So this should be good in my view.

I wonder if we can set this hint regardless of platform? If we can, I would prefer this. Do you know if there are any situations where forward compatible context could fail, where it would otherwise succeed? I briefly tested it locally (windows/nvidia), and it works well for me.

If you cannot find any counter-examples, then I'd appreciate removing the if condition, and we can instead add it back if other users report any issues.

[NaLiJa]

Yes, it should be save to remove the Apple tag if RmlUi is not using any of the deprecated featries! Have changed the PR accordingly!

[mikke89]

Perfect, thank you for contributing :)