wayland: Represent preedit string cursor offset in bytes
Both IBus and ClutterInputFocus work in character offsets for the cursor position in the preedit string. However the zwp_text_input protocol does define the preedit string cursor offset to be in bytes.
Fixes client bugs in representing the caret within the preedit string, as we were clearly giving the wrong offset.
Fixes: gtk#2517 (closed)
Edited by Carlos Garnacho