remove indirection in vertex builder

src/rendering/twn_billboards.c

@@ -125,9 +125,11 @@ void finally_draw_billboard_batch(struct MeshBatch const *batch,
                 .v3 = vec3_add(billboard.position, a),
             };
 
-            push_to_vertex_buffer_builder(&builder, &payload, sizeof (payload));
+            ((struct ElementIndexedBillboard *)builder.base)[i] = payload;
         }
 
+        finish_vertex_builder(&builder);
+
         /* commit to drawing */
         DeferredCommandDraw command = {0};