data-device: avoid duplicating data offer for the same client
`weston_data_source_send_offer` is called to create a new data offer every time when the seat focuses a surface. This patch will evite create a new data offer for the same client for the same source, it fixes the bug of #376, where the data device duplicates data offers thus it never been pasted to firefox. Signed-off-by: xichen zhou <sichem.zh@gmail.com>