Attempt to account for OpenExrFile's new way of handling utf-8 filenames

See https://github.com/AcademySoftwareFoundation/openexr/issues/292 ,
https://github.com/AcademySoftwareFoundation/openexr/blob/master/src/lib/OpenEXR/ImfStdIO.cpp#L116
and https://forum.kde.org/viewtopic.php?f=139&t=168870&p=440067#p440067

We need to be aware, though, that this still might blow up on us.

CCMAIL:kimageshop@kde.org