Keep decoder reserve based on paced probe rather than transient burst depth

tools/chatgui/src/call_video_decoder.cpp

@@ -177,7 +177,10 @@ int CallVideoDecoder::receive(call_video_frame& f, QImage& image) {
     }
     av_frame_free(&copied); av_frame_free(&decoded);
     if (result < 0) return m->failed("output PTS/profile/geometry/transfer", result);
-    if (!m->outputSeen) { m->pipeline = static_cast<unsigned>(m->pending.size() - 1); m->outputSeen = true; }
+    if (!m->outputSeen) {
+        if (m->backend == Backend::HardwareOnly) m->pipeline = static_cast<unsigned>(m->pending.size() - 1);
+        m->outputSeen = true;
+    }
     m->pending.pop_front(); return 1;
 #else
     Q_UNUSED(f); Q_UNUSED(image); return -1;