Add commenting to bezctx_ff.c from http://libspiro.sourceforge.net/

Also converted chunkalloc() and chunkfree() to generic calloc() and free().
issue 605, fix splineoverlap.c:2146 always evaluates to true

Used with Element->Overlap->Intersect
Merge pull request #613 from JoesCat/master

Add commenting to bezctx_ff.c from http://libspiro.sourceforge.net