Treat pattern name as UTF-8 when loading a .pat resource

GIMP pattern names are stored as null-terminated UTF-8 strings, but we were loading them as Latin-1.

Fixes bug 453637

References:

• (UTF-8) https://developer.gimp.org/core/standards/pat/
• (Null-termination) https://gitlab.gnome.org/GNOME/gimp/-/blob/127cd67118727ef9086c829e66b7894c72c2ef98/app/core/gimppattern-header.h#L42

Formalities Checklist

(I didn't see relevant unit tests, but if I missed one, let me know.)

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.