For text objects, the CharInfo mat_nr material index used to start at 1
(not at zero like for meshes or nurbs).
Code was mostly considering this (but not in all places, so material
index handling (removing/moving) could still go wrong.
As an alternative to !109746 (where it was made sure all places would
make the right assumption about mat_nr starting at 1), this PR now
changes the mat_nr to be zero-based.
This is more in line with other places handling material indices.
Versioning code is in place to properly convert old files.
Fixes#109491
Philipp Oeser