fix intro/outro producing mono audio

src/render/filter.rs

@@ -191,7 +191,11 @@ impl Filter {
 					channel('v', video),
 					channel('v', output)
 				);
-				writeln!(complex, "aevalsrc=0:s=48000{};", channel('a', output));
+				writeln!(
+					complex,
+					"aevalsrc=0:s=48000,pan=stereo|c0=c0|c1=c0{};",
+					channel('a', output)
+				);
 			},
 
 			Self::FastForward {