fmp4mux: Don't write the first sample flags into any trun but the first

The flags are based on the first sample of this fragment so writing it
into any trun but the first is not very useful.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1060>

mux/fmp4/src/fmp4mux/boxes.rs

@@ -1839,7 +1839,7 @@ fn write_traf(
     // has to be stored for every single sample
     let (
         tf_flags,
-        tr_flags,
+        mut tr_flags,
         default_size,
         default_duration,
         default_flags,
@@ -1895,6 +1895,9 @@ fn write_traf(
         current_data_offset = (current_data_offset as u64
             + run.iter().map(|b| b.buffer.size() as u64).sum::<u64>())
         .try_into()?;
+
+        // Don't include first sample flags in any trun boxes except for the first
+        tr_flags &= !FIRST_SAMPLE_FLAGS_PRESENT;
     }
 
     // TODO: saio, saiz, sbgp, sgpd, subs?