snap: cache and ignore last-snapped edge when growing or shrinking

When growing or shrinking a view by snapping to an edge, a client may
ignore the requested size and instead keep its original size or
substitute a different (possibly constrained) size. In this case, the
view may not actually contact the snapped edge, and a subsequent snap
attempt will just keep re-trying (and failing) to contact the same ege.

To mitigate this, remember the last-snapped view, snapping direction and
offset of the snapping edge in snap.c; when re-attempting a snap for the
same view in the same direction, ignore the edge that was last "hit", to
allow snapping to progress beyond the problematic edge.

src/meson.build

@@ -21,6 +21,7 @@ labwc_sources = files(
   'seat.c',
   'server.c',
   'session-lock.c',
+  'snap-constraints.c',
   'snap.c',
   'tearing.c',
   'theme.c',