fix(maps): set_cell_material invalidates _dirty + _opaque_ceiling

Mirrors the cache-invalidation that set_vertex and set_cell_gid
do. Without this, a cell-tile material write would update the
storage but the opaque-ceiling cache could continue to report a
stale state until something else invalidated it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -1600,6 +1600,8 @@ function M.set_cell_material(layer_name, cx, cy, painted, map_id)
     end
     local cm = ensure_cells_material(layer, w, h)
     cm[cy * w + cx + 1] = painted and 1 or 0
+    m._dirty = true
+    m._opaque_ceiling = nil
 end
 
 -- 0.5.7: read the per-cell material flag. Returns false if