Slow opening of exr image in GIMP

[haaninjo]

Over in GIMP we got a report of .exr images being slow to load recently: https://gitlab.gnome.org/GNOME/gimp/-/issues/12169

Could reproduce this and see that it didn't happen when using an older version of openexr, so bisected this, and got this result:

7e0da7f5b79902e5a8e38f227d8e6f36cc3ef655 is the first bad commit
commit 7e0da7f5b79902e5a8e38f227d8e6f36cc3ef655 (HEAD)
Author: Kimball Thurston <[email protected]>
Date:   Sun Apr 28 12:01:36 2024 +1200

    Convert scanline input file to use the core
    
    simplifies input file as well. Future changes may lift common code out
    of scanline input into common utility in context / somewhere.

Test case: Exported https://img.photographyblog.com/reviews/nikon_d850/photos/nikon_d850_21.nef to exr in Darktable, and opened the resulting image in GIMP. On earlier versions of openexr it took about 4 seconds, while it takes 50 seconds after commit 7e0da7f5b79902e5a8e38f227d8e6f36cc3ef655.