fmp4mux: Write lmsg as compatible brand into the last fragment

While this is only defined as such in DASH, it's used more widely and
shouldn't hurt to always have it there.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2222>

mux/fmp4/src/fmp4mux/boxes.rs

@@ -1818,11 +1818,15 @@ pub(super) fn create_fmp4_fragment_header(
 ) -> Result<(gst::Buffer, u64), Error> {
     let mut v = vec![];
 
-    // Don't write a `styp` if this is only a chunk.
-    if !cfg.chunk {
-        let (brand, compatible_brands) =
+    // Don't write a `styp` if this is only a chunk unless it's the last.
+    if !cfg.chunk || cfg.last_fragment {
+        let (brand, mut compatible_brands) =
             brands_from_variant_and_caps(cfg.variant, cfg.streams.iter().map(|s| &s.caps));
 
+        if cfg.last_fragment {
+            compatible_brands.push(b"lmsg");
+        }
+
         write_box(&mut v, b"styp", |v| {
             // major brand
             v.extend(brand);

mux/fmp4/src/fmp4mux/imp.rs

@@ -3211,6 +3211,7 @@ impl FMP4Mux {
                 chunk: !fragment_start,
                 streams: streams.as_slice(),
                 buffers: interleaved_buffers.as_slice(),
+                last_fragment: at_eos,
             })
             .map_err(|err| {
                 gst::error!(

mux/fmp4/src/fmp4mux/mod.rs

@@ -240,6 +240,9 @@ pub(crate) struct FragmentHeaderConfiguration<'a> {
 
     streams: &'a [FragmentHeaderStream],
     buffers: &'a [Buffer],
+
+    /// If this is for the last fragment.
+    last_fragment: bool,
 }
 
 #[derive(Debug)]