octave: resolve "GraphicsMagick limits images to X bits per pixel" warning

[mcandre]

When I try to imread() a high quality .jpg, my brew-built octave complains

warning: your version of GraphicsMagick limits images to 16 bits per pixel

Please ./configure --with-quantum-depth=32 when compiling graphicsmagick, so that it can handle rich images.

See http://wiki.octave.org/GraphicsMagick for more information.

[UniqMartin]

You can fix that yourself easily by rebuilding GraphicsMagick with brew reinstall graphicsmagick --with-quantum-depth-32. I'm not sure if it is feasible to change the default for all users of the graphicsmagick formula, as the higher bit depth certainly comes with more computational overhead and few applications actually need this. You can read some of the discussion in #46101, #46248, and #47014 that resulted in 16 bits being the default (was 8 bits before).

[DomT4]

Closing per the above. I believe QD16 is the default upstream, so it's possibly better to raise this with them if it's a common issue.