fix side indices & update test model

res/models/stairs.xml

@@ -1,5 +1,6 @@
 <model>
-    <box from="0,0,0" to="1,0.5,1" delete="top"/>
-    <box from="0,0.5,0.5" to="1,1,1" delete="bottom"/>
+    <box from="0,0,0" to="1,0.5,1" delete="top,north"/>
+    <box from="0,0.5,0.5" to="1,1,1" delete="bottom,north"/>
     <rect from="0,0.5,0" right="1,0,0" up="0,0,0.5"/>
+    <rect from="1,0,1" right="-1,0,0" up="0,1,0"/>
 </model>

src/coders/vcm.cpp

@@ -10,12 +10,12 @@ using namespace vcm;
 using namespace xml;
 
 static const std::unordered_map<std::string, int> side_indices {
-    {"east", 0},
-    {"west", 1},
+    {"north", 0},
+    {"south", 1},
     {"top", 2},
     {"bottom", 3},
-    {"back", 4},
-    {"front", 5},
+    {"east", 4},
+    {"west", 5},
 };
 
 static void perform_rect(const xmlelement& root, model::Model& model) {