fmp4mux: Fix decision whether per-sample flags are needed in the trun

Previously it would never use per-sample flags if any later sample
needed different flags than the first two.

Also comment the code a bit better.

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

mux/fmp4/src/fmp4mux/boxes.rs

@@ -1760,15 +1760,19 @@ fn analyze_buffers(
 
         let f = sample_flags_from_buffer(stream, buffer);
         if first_buffer_flags.is_none() {
+            // First buffer, remember as first buffer flags
             first_buffer_flags = Some(f);
-        } else {
+        } else if flags.is_none() {
+            // Second buffer, remember as general flags and if they're
+            // different from the first buffer's flags then also remember
+            // that
             flags = Some(f);
             if Some(f) != first_buffer_flags {
                 tr_flags |= FIRST_SAMPLE_FLAGS_PRESENT;
             }
-        }
+        } else if Some(f) != flags {
-
+            // Third or later buffer, and the flags are different than the second buffer's flags.
-        if flags.is_some() && Some(f) != flags {
+            // In that case each sample will have to store its own flags.
             tr_flags &= !FIRST_SAMPLE_FLAGS_PRESENT;
             tr_flags |= SAMPLE_FLAGS_PRESENT;
         }
@@ -1803,6 +1807,8 @@ fn analyze_buffers(
         flags = first_buffer_flags.take();
     }
 
+    // If all but possibly the first buffer had the same flags then only store them once instead of
+    // with every single sample.
     if (tr_flags & SAMPLE_FLAGS_PRESENT) == 0 {
         tf_flags |= DEFAULT_SAMPLE_FLAGS_PRESENT;
     } else {