Added OpenGL 3.0 context creation API entries

Added OpenGL 3.0 context creation API entries

   glutInitContextVersion,
   glutInitContextFlags

and their related constants

   GLUT_INIT_MAJOR_VERSION
   GLUT_INIT_MINOR_VERSION
   GLUT_INIT_FLAGS

   GLUT_DEBUG
   GLUT_FORWARD_COMPATIBLE

Note that this works with GLX only currently, the glutInitContext* API entries
have no effect for WGL yet.

TODO: Centralize the context creation code for WGL (the harder part) and use the
new wglCreateContextAttribsARB API entry (the easy part, re-use most of the GLX
code).

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@749 7f0cb862-5218-0410-a997-914c9d46530a