Make PNG converter check file chunk for Blender 16bit pngs

If Blender writes the relevant blend file to 'File', we know it's
a blender png, and we can assume it is sRGB. This'll smoothen the
workflow between the two.

Fixes T1077 together with the previous commit, until we find a program
that isn't Krita writing to the gAma chunk.
Differential Revision: https://phabricator.kde.org/D1228