Cleanup pass after committing callback conversion (from structure to

Cleanup pass after committing callback conversion (from structure to
array-of-pointers).  Mostly deleted commented-out struct-based code.
Also added some XXX comments that I though should be pondered.  (My
own pair of eyes, and one brain cell, were not sufficient at this time
to decide what to do for those ponderables.  (^&)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@304 7f0cb862-5218-0410-a997-914c9d46530a