text input : mismatch between QString and byte counts in protocols

There are some mismatches in conversions between QString size and byte counts in textinput protocols.
I tested text-input-v2 and text-input-v3 but text-input-v1 might also have the same problem.

Using qtvirtualkeyboard enabeld hunspell in "basic" example.

When clicking random words, some of the text are modified or added and frequently the client crashes.