rectpack: add_rect() in online packers doesn't return solution

I am using the online packer and calling add_rect() when needed. However, the return value is inconsitent and never complete. The only way to get the full solution seems to be to iterate all bins which is O(n) for n rects packed so far. The solution is available and could be returned, but…

• BBF returns bool indicating whether packing succeeded.
• BNF and BBF return a rect if successful or None otherwise - but not the bin ID it was packed into.

I would suggest changing the return value for all three to a tuple of the form (bin_id, rect).