aplay/pipewire: Set buffer frame count

This is optional and doesn't do anything by itself, but it's returned by
pipwire to the application in the pw_time struct as 'queued'.

Author: mpiatka

src/audio/playback/pipewire.cpp

@@ -130,6 +130,8 @@ static void on_process(void *userdata) noexcept{
                 buf->datas[0].chunk->stride = frame_size;
                 buf->datas[0].chunk->size = to_write_total * frame_size;
 
+                b->size = to_write_total;
+
                 pw_stream_queue_buffer(s->stream.get(), b);
         }
 }