vf_tonemap: Fix logic for detecting the maximum peak of untagged sources

When there is no metadata attached to a frame, take into account both
the PQ and HLG transfers, and change the HLG default value to 10:
the value of 12 is the maximum range in scene referred light, but
the reference OOTF maps this from 0 to 1000 cd/m² on the ideal HLG
monitor.

This matches what vf_tonemap_opencl does.

libavfilter/vf_tonemap.c

@@ -131,10 +131,10 @@ static double determine_signal_peak(AVFrame *in)
             peak = av_q2d(metadata->max_luminance) / REFERENCE_WHITE;
     }
 
-    /* smpte2084 needs the side data above to work correctly
-     * if missing, assume that the original transfer was arib-std-b67 */
+    // For untagged source, use peak of 10000 if SMPTE ST.2084
+    // otherwise assume HLG with reference display peak 1000.
     if (!peak)
-        peak = 12;
+        peak = in->color_trc == AVCOL_TRC_SMPTE2084 ? 100.0f : 10.0f;
 
     return peak;
 }