twn_lines.c: introduction with proper impl

src/rendering/twn_deferred_commands.h

@@ -4,6 +4,7 @@
 #include "twn_types.h"
 #include "twn_gpu_texture_c.h"
 #include "twn_textures_c.h"
+#include "twn_types_c.h"
 
 #include <stddef.h>
 #include <stdbool.h>
@@ -49,6 +50,11 @@ typedef struct {
     uint32_t element_count;
     uint32_t range_start, range_end;
 
+    enum {
+        DEFERRED_COMMAND_DRAW_GEOMETRY_MODE_TRIANGLES = 0,
+        DEFERRED_COMMAND_DRAW_GEOMETRY_MODE_LINES = 1,
+    } geometry_mode;
+
     bool constant_colored;
     bool textured, texture_repeat, uses_gpu_key;
 } DeferredCommandDraw;