wayland: Add possibility to get the exported handle multiple times
@jadahl
Submitted by Jonas Ådahl Link to original bug (#782325)
Description
Extends the API in a backward compatible way where the caller can call export multiple times, getting the same exported handle.
For gtk4 we should consider changing the API some how. For example adding a handle to the unexport means we can export the same surface multiple times with unique handles. If we add a intermediate object, we can add the abilitiy to unexport before having received the handle (though this would of course make the API more complicated).