1.0.6: conditional pins, Pokemon Tower, plateau statues, cave elevation

Adds `when_above` conditional pins, resolved per position in
TileShape.at: one graphic can mean two things (the gates' $32 is both
wall base course and counter front), and a flat pin has to pick one.
Gates get half-height counters and level walls.

Pokemon Tower gains a buildings entry -- it is the drawing the map edge
cuts off, sealed on the north -- and the Indigo Plateau statues are
built like the gym statues.

Fixes: grass stands one full-height clump per tile instead of two
half-cut stubs at different depths; ledge pillars drop to ledge height;
a prop only rides furniture when its own cell is blocked (chairs were
being lifted onto tables); and the caves had water and rock pinned
backwards, which stood the Seafoam sea up as rock and cut trenches
through Mt Moon. No cave tile is below the datum now.

lib/Structures.lua

@@ -1919,10 +1919,24 @@ function Structures.buildObject(S, map, region, cluster,
   -- the box's top with its feet on the box's north row, and the claimed
   -- tiles keep rendering as that box (wearing its plain art) instead of
   -- punching a floor-level hole through it.
+  --
+  -- Only when the prop's OWN CELL IS BLOCKED, though.  "Is something
+  -- drawn above me?" is not the same question as "am I standing on it":
+  -- a chair drawn against the north side of a table is above the table's
+  -- trim row too, and it was being lifted onto the tabletop -- three
+  -- chairs standing on the furniture in Cinnabar's trade room and
+  -- Fuchsia's meeting room, with the claimed cells re-tiled as tabletop
+  -- so the table marched two rows north with them.  The world already
+  -- knows which is which: a thing that sits ON furniture occupies a
+  -- blocked cell (you cannot walk through the gym statue, Red's plant,
+  -- the PC), while a seat you walk up to is in a walkable one.
   local baseY, support = 0, nil
   if force then
     local bs = S.shapeAt[keyOf(cluster.minX, cluster.maxY + 1)]
-    if bs and bs.authored and bs.art == "upright" and (bs.h or 0) > 0 then
+    local blocked = not map:isWalkableCell(math.floor(cluster.minX / 2),
+                                           math.floor(cluster.maxY / 2))
+    if blocked and bs and bs.authored and bs.art == "upright"
+       and (bs.h or 0) > 0 then
       baseY, support = bs.h, bs
     end
   end