[Examples] Improved ExternalContext test

Also added documentation
[OpenTK] Fixed DllImport library for OpenGL on Mac OS X
[Bind] Do not generate slots for OpenGL <= v1.1

We can use DllImports for OpenGL functions <= v1.1 on all platforms,
including Windows. This allows us to reduce the number of required
GetProcAddress calls by 474, reducing startup time and memory
consumption.

This setting can be turned off through Settings.Legacy.UseDllImports
and UseWindowsCompatibleGL.