support for offscreen rendering with OSMesa was unavailable

[Francesco Potortì]

Out of the blue, without any apparent reason, my script stopped working
with this error:

error: __osmesa_print__: support for offscreen rendering with OSMesa was 
unavailable or disabled when Octave was built
error: called from
    __opengl_print__ at line 161 column 7
    print at line 561 column 14

This is Octave 4.4.0 on Debian testing

Before starting to investigate, is there anything obvious or well known
that I'm missing?

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