Finished off most of the issues with freeglut_structure.c, from a stylistic

Finished off most of the issues with freeglut_structure.c, from a stylistic
point of view (at least, insofar as: The original file's code was INCON-
SISTANT.  I did not remove the "!= NULL" stuff, did not address the
shortest-branch-first issue for if-else statements, and left some rather
ugly "if (x) {... return y} /* else do this */ return NULL;" garbage.
This should, I think, be re-written as "if (x) return y; else return NULL;"
or even better, "ret = NULL; if (x) ret = y; return ret;"

In short, the code still has some issues, but I think that it's a bit
better.

(Oh, I also got rid of oustanding TABs.)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@271 7f0cb862-5218-0410-a997-914c9d46530a