Added fgInitCallBacks() to freeglut_structure.c. (The function takes

Added fgInitCallBacks() to freeglut_structure.c.  (The function takes
an {SFG_Window *}.)  Added prototype to freeglut_internal.h.  Added use
of the function to the window initialization.  (Don't count on calloc(),
which sets all bits to 0.)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@316 7f0cb862-5218-0410-a997-914c9d46530a