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>
3 jobs for fix_data_device_for_firefox in 1 minute and 21 seconds
latest
Status Job ID Name Coverage
  Container Prep
passed #2720456
container_prep

00:00:26

 
  Build
passed #2720457
build-native-meson-default-options

00:00:55

passed #2720458
build-native-meson-no-gl-renderer

00:00:34