wayland/popup: Handle late received reposition feedback

wayland/popup: Handle late received reposition feedback

If we map, reposition, unmap, remap, the reposition feedback from the
last time a popup was mapped might be received while we're dealing with
the new version of the popup. At this point, the old reposition token
has no meating, so lets drop it. Also reset the reposition tokens when
creating new protocol objects, so that the reposition token are as if
we're in the initial state.

This fixes an issue where we'd get stuck if repeatedly smashing a button
that'd create popups that'd immediately get dismissed by the compositor.