some cleanup of one demo. added menu back in gamemode

some cleanup of one demo. added menu back in gamemode
Now proper animation whose speed does not depend on the number of
windows currently open
Got some flicker in gamemode when other windows (that weren't visible)
were also updated (only occurs when child windows present). Simply no
drawing to these windows fixes things - one shouldn't draw anyway as
they aren't visible.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1366 7f0cb862-5218-0410-a997-914c9d46530a