mp4mux: Don't write gap edit lists if their duration would be zero

The track might start later than the earliest track by less than one
timescale units, in which case writing an empty gap edit list would be
useless and confusing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1017>

mux/mp4/src/mp4mux/boxes.rs

@@ -1646,24 +1646,30 @@ fn write_elst(
     let min_earliest_pts = header.streams.iter().map(|s| s.earliest_pts).min().unwrap();
 
     if min_earliest_pts != stream.earliest_pts {
-        // Entry count
-        v.extend(2u32.to_be_bytes());
-
-        // First entry for the gap
-
-        // Edit duration
         let gap = (stream.earliest_pts - min_earliest_pts)
             .nseconds()
             .mul_div_round(timescale as u64, gst::ClockTime::SECOND.nseconds())
             .context("too big gap")?;
-        v.extend(gap.to_be_bytes());
 
-        // Media time
-        v.extend((-1i64).to_be_bytes());
+        if gap > 0 {
+            // Entry count
+            v.extend(2u32.to_be_bytes());
 
-        // Media rate
-        v.extend(1u16.to_be_bytes());
-        v.extend(0u16.to_be_bytes());
+            // First entry for the gap
+
+            // Edit duration
+            v.extend(gap.to_be_bytes());
+
+            // Media time
+            v.extend((-1i64).to_be_bytes());
+
+            // Media rate
+            v.extend(1u16.to_be_bytes());
+            v.extend(0u16.to_be_bytes());
+        } else {
+            // Entry count
+            v.extend(1u32.to_be_bytes());
+        }
     } else {
         // Entry count
         v.extend(1u32.to_be_bytes());