use static, fixed arrays for circle geometry data instead of allocating for each one

src/rendering/twn_draw_c.h

@@ -126,12 +126,14 @@ void render(void);
 /* clears all render queues */
 void render_queue_clear(void);
 
+/* fills two existing arrays with the geometry data of a circle */
+/* the size of indices must be at least 3 times the number of vertices */
 void create_circle_geometry(Vec2 position,
                             Color color,
                             float radius,
                             size_t num_vertices,
-                            SDL_Vertex **vertices_out,
-                            int **indices_out);
+                            SDL_Vertex vertices[],
+                            int indices[]);
 
 struct SpriteBatch {
     size_t size;             /* how many primitives are in current batch */