gtk text input v3 delete surrounding text implemenation using parameter in bytes as number of characters.

Quote from text-input-v3 protocol: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/main/unstable/text-input/text-input-unstable-v3.xml#L388 "Before_length and after_length are the number of bytes before and after the current cursor index (excluding the selection) to delete."

Quote from gtk_im_context_delete_surrounding doc "offset and n_chars are in characters not in bytes which differs from the usage other places in GtkIMContext."

GtkIMContextWayland didn't do the necessary conversion between characters and bytes.