gtkimcontextwayland: Convert byte based offset to char based offset in delete_surroun...

gtkimcontextwayland: Convert byte based offset to char based offset in delete_surrounding_text

ClutterInputFocus/GtkIMContext uses char based offset for
delete_surrounding, however, text_input_v3 uses byte based offset for
it. Currently only GTK with mutter can work correctly via text_input_v3
because they both forget to convert between char based offset and byte
based offset.

This commit fixes it in GTK by converting byte based offset to char
based offset with the UTF-8 encoded surrounding text.

Fixes <https://gitlab.gnome.org/GNOME/gtk/-/issues/4566>.