MacOS: use common OpenGL/AL codepath (1/2)

This PR switches MacOS from MonoMac.OpenGL/AL to the corresponding OpenTK namespaces. This allows us to use a common codepath for the WinGL/Linux/MacOS platforms, which reduces the support burden (see comments in #2782.)

Changes:
- `MonoMac.OpenGL/AL` namespaces converted to `OpenTK.Graphics.OpenGL` and `OpenTK.Audio.OpenAL` respectively.
- `#if MONOMAC` regions regarding API differences between MonoMac and OpenTK are eliminated.
- `MacGamePlatform` and `GameWindow` are not affected - we are still using MonoMac for platform support.
- Unit tests remain unaffected (87/166 passing on my system.)

This PR does not introduce any functional differences and should be safe to apply as-is. I'll issue a separate PR for switching `MacGamePlatform` to `OpenTKGamePlatform`, as that is much more invasive change and should be reviewed separately.