Adopted Eero Pajarre's suggestion of using a do { ... } while

Adopted Eero Pajarre's suggestion of using a do { ... } while
rather than { ... } for the INVOKE_WCB() macro.  This lets it
be used "more like a function", in that:

if( ... )
    INVOKE_WCB( ... );
else
    ...

...is now legal.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@321 7f0cb862-5218-0410-a997-914c9d46530a