Add watermark to docx

[Daniel Torregrosa Rivero]

Hi all,

I have been using the code at https://stackoverflow.com/a/48250236 to
insert watermarks in docx documents. But when I upgraded POI to 5.0, this
stopped working. In particular, getPictList()/getPictArray() returns an
empty list now.

Does anybody have an updated recipe to add a watermark?

Thanks a lot,
Daniel