[Lazarus] Opposite of UTF8CharToByteIndex

[Ondrej Pokorny]

I am struggling to find the opposite of UTF8CharToByteIndex from 
LazUTF8. I.e. I need to convert ByteIndex to Char index. Is there such a 
function? Or do I have to write one by myself? (Isn't a problem, I'd 
like to check first.)

Ondrej