wayland: Set character length for set_surrounding_text
Convert byte offsets to the character length in text_input_set_surrounding_text() since text-input protocol defines the byte offsets but IBus handles the character length.
Fixes: #3102
Convert byte offsets to the character length in text_input_set_surrounding_text() since text-input protocol defines the byte offsets but IBus handles the character length.
Fixes: #3102