Added "offscreen" demo, a very simple program that produces an onscreen

Added "offscreen" demo, a very simple program that produces an onscreen
window and, upon command, renders a similar offscreen display and writes
the result to disk.

Also, modified the build structure for UNIX_X11 to autobuild the demo.
(Not done for WIN32 at this time.)

Also, forgot to previously commit the updated freeglut_ext.h include.
Eeep.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@465 7f0cb862-5218-0410-a997-914c9d46530a