OpenGL on MacOSX: update AppleGLGetProcAddress() to use dlsym
OpenGL on MacOSX: update AppleGLGetProcAddress() to use dlsym

Default implementation using NSLookupSymbolInImage() doesn't seem to
compile on MacOSX Mavericks.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3