twn_lines.c: 3d case

src/twn_engine_context_c.h

@@ -50,6 +50,13 @@ typedef struct EngineContext {
     MeshBatchItem *uncolored_mesh_batches;
     MeshBatchItem *billboard_batches;
     MeshBatchItem *quad_batches;
+    struct LineBatchItem {
+        struct LineBatchItemKey {
+            Color color;
+            uint8_t thickness;
+        } key;
+        struct LinePrimitive value;
+    } *line_batches;
     TextCache text_cache;
     TextureCache texture_cache;