fix caves issues & add block 'surface-replacement' property

src/content/ContentBuilder.cpp

@@ -82,6 +82,7 @@ std::unique_ptr<Content> ContentBuilder::build() {
     // Now, it's time to resolve foreign keys
     for (Block* def : blockDefsIndices) {
         def->rt.pickingItem = content->items.require(def->pickingItem).rt.id;
+        def->rt.surfaceReplacement = content->blocks.require(def->surfaceReplacement).rt.id;
     }
 
     for (ItemDef* def : itemDefsIndices) {

src/content/ContentLoader.cpp

@@ -286,6 +286,7 @@ void ContentLoader::loadBlock(
     root.at("hidden").get(def.hidden);
     root.at("draw-group").get(def.drawGroup);
     root.at("picking-item").get(def.pickingItem);
+    root.at("surface-replacement").get(def.surfaceReplacement);
     root.at("script-name").get(def.scriptName);
     root.at("ui-layout").get(def.uiLayout);
     root.at("inventory-size").get(def.inventorySize);