twn_gl_15_rendering.c: fix endlessly generating display lists

src/rendering/twn_gl_15_rendering.c

@@ -201,7 +201,7 @@ void render_circle(const CirclePrimitive *circle) {
 
 
 void use_texture_mode(TextureMode mode) {
-    GLuint lists = 0;
+    static GLuint lists = 0;
     if (!lists) {
         lists = glGenLists(3);