Bug#1018015: src:ilmbase will be removed

[Mathieu Malaterre]

Source: openimageio
Version: 2.3.18.0+dfsg-3
Severity: serious

I am planning to remove ilmbase now that openexr 3.1.5 has migrated to
testing. Please replace dependency from  libilmbase-dev into
libimath-dev. There should not be any other difference.