Use a callback to notify frame skip

A skipped frame is detected when the producer offers a frame while the
current pending frame has not been consumed.

However, the producer (in practice the decoder) is not interested in the
fact that a frame has been skipped, only the consumer (the renderer) is.

Therefore, notify frame skip via a consumer callback. This allows to
manage the skipped and rendered frames count at the same place, and
remove fps_counter from decoder.

app/src/scrcpy.c

@@ -346,7 +346,7 @@ scrcpy(const struct scrcpy_options *options) {
             file_handler_initialized = true;
         }
 
-        decoder_init(&decoder, &video_buffer, &fps_counter);
+        decoder_init(&decoder, &video_buffer);
         dec = &decoder;
     }