# [Jelly], 7 bytes

    ṚZ$ÐĿ+/

[Try it online!][TIO-jcn91z18]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-jcn91z18]: https://tio.run/##y0rNyan8///hzllRKocnHNmvrf/////oaEMTHQUjHQVTHQVDAyArVkch2tBCR8ESyAcKGwJZICFjMBOkCEgZmoBVAcWMgFrMgQJg1SBBqBawUrPYWAA "Jelly – Try It Online"

An alternative to [Leaky Nun's answer](https://codegolf.stackexchange.com/a/153664/59487).

###How?

<pre>
ṚZ$ÐĿ+/ || Full program (monadic).

   ÐĿ   || Until the results are no longer unique, collect them in a list.
  $     || Apply the last two links as a monad:
Ṛ       || ... Reverse, then...
 Z      || ... Transpose
     +/ || Reduce by (vectorised) addition.
</pre>