[PATCH] add missing lock/unlock in unicapgtk_video_display_set_pause()

The attached patch adds missing lock/unlock in unicapgtk_video_display_set_pause(). We have a crasher bug in our bugzilla, reported half a year ago:

https://bugzilla.redhat.com/show_bug.cgi?id=532289

Looking at the code, I discovered that the place where the crash occurs is the only place where ugtk->backend_get_image_data() is called without calling ugtk->backend_lock() first. As long as I understand the code, this must be a mistake. However I haven't been successful in reproducing the crash myself and the fix is not yet confirmed by users.

Could you please have a look at the attached patch? Thanks in advance!