vwebp/animation: display last frame on end-of-loop

previously the first frame would be redisplayed, which might be
unexpected if the final frame was meant to be a composite, for example.

(cherry picked from commit 0f017b56f3995cc7439bd62b0980eb1cc56c856e)

Change-Id: I4da795623c71501e2fa426e8fba8fb2ffcbab58a

examples/vwebp.c

@@ -181,6 +181,7 @@ static void decode_callback(int what) {
         if (WebPDemuxGetFrame(kParams.dmux, 1, curr)) {
           --kParams.loop_count;
           kParams.done = (kParams.loop_count == 0);
+          if (kParams.done) return;
         } else {
           kParams.decoding_error = 1;
           kParams.done = 1;