Assign PTS to the right frame

The PTS was read from the socket and set as the current one even before
the frame was consumed, so it could be assigned to the previous frame
"in advance".

Store the PTS for the current frame and the last PTS read from the
packet header of the next frame in separate fields.

As a side-effect, this fixes the warning on quit:

> Application provided invalid, non monotonically increasing dts to
> muxer in stream 0: 17164020 >= 17164020

app/src/decoder.h

@@ -15,6 +15,7 @@ struct decoder {
     SDL_Thread *thread;
     SDL_mutex *mutex;
     struct recorder *recorder;
+    uint64_t next_pts;
     uint64_t pts;
     uint32_t buffer_info_flags;
     int remaining;