Set video length to 7.1 seconds, mono audio

generate.sh

@@ -96,6 +96,7 @@ render_video() {
   -framerate 50 -i frame-%d.webp \
   -i ${SOURCEDIR}/input/bumper.m4a \
   -map 0:v:0 -map 1:a:0 \
+  -t 7.1 \
   -c:v libx264 \
   -profile:v main \
   -pix_fmt yuv420p \
@@ -105,8 +106,7 @@ render_video() {
   -color_range tv \
   -r 50 \
   -c:a aac \
-  -ac 2 \
-  -af "pan=stereo|c0=c0|c1=c0,apad" \
+  -af "apad" \
   "${VIDEODIR}/${PROJECT}-x264.mkv"
 
 }