Placeholder tile for primary layer when requesting out-of-bounds tile

tileset.cpp

@@ -40,7 +40,11 @@ QImage Metatile::getMetatileImage(int tile, Tileset *primaryTileset, Tileset *se
         if (tile_image.isNull()) {
             // Some metatiles specify tiles that are outside the valid range.
             // These are treated as completely transparent, so they can be skipped without
-            // being drawn.
+            // being drawn unless they're on the bottom layer, in which case we need
+            // a placeholder.
+            if (layer == 0) {
+                metatile_painter.fillRect(x * 8, y * 8, 8, 8, palettes.value(0).value(0));
+            }
             continue;
         }