Fix memory leak when VNC exit

- Register client cleanup function when during initialization to ensure
that twin_peer_t is freed properly when the client disconnects.
- Properly release resources during VNC server shutdown by unreferencin
current_fb and free the framebuffer allocated for tx->framebuffer

Close #76

apps/multi.c

@@ -268,6 +268,7 @@ static void apps_flower_start(twin_screen_t *screen, int x, int y, int w, int h)
     draw_flower(path, D(3), 5);
     twin_paint_path(pixmap, 0xffe2d2d2, path);
     twin_path_destroy(stroke);
+    twin_path_destroy(path);
     twin_window_show(window);
 }
 

backend/vnc.c

@@ -102,10 +102,18 @@ static bool _twin_vnc_work(void *closure)
     return true;
 }
 
+static void _twin_vnc_client_cleanup(struct nvnc_client *client)
+{
+    twin_peer_t *peer = nvnc_get_userdata(client);
+    free(peer);
+    nvnc_set_userdata(client, NULL, NULL);
+}
+
 static void _twin_vnc_new_client(struct nvnc_client *client)
 {
     twin_peer_t *peer = malloc(sizeof(twin_peer_t));
     nvnc_set_userdata(client, peer, NULL);
+    nvnc_set_client_cleanup_fn(client, _twin_vnc_client_cleanup);
 }
 
 static void _twin_vnc_pointer_event(struct nvnc_client *client,
@@ -268,11 +276,12 @@ static void twin_vnc_exit(twin_context_t *ctx)
         return;
 
     twin_vnc_t *tx = PRIV(ctx);
-
+    nvnc_fb_unref(tx->current_fb);
     nvnc_display_unref(tx->display);
     nvnc_close(tx->server);
     aml_unref(tx->aml);
 
+    free(tx->framebuffer);
     free(ctx->priv);
     free(ctx);
 }