Docker: Respect SE_SCREEN_{WIDTH,HEIGHT} variables in ffmpeg recording

Currently, these variables are always overridden in the call the
wait_for_display(). Before the upgrade to ffmpeg-7.0.2 (PR SeleniumHQ#2374), the
SE_SCREEN_WIDTH and SE_SCREEN_HEIGHT variables were respected in the
case where no video uploading was done because wait_for_display() was
never called. After the upgrade to 7.0.2, it the function is always
called and overrides the VIDEO_SIZE variable.

Now, if both screen width and height are specified, the VIDEO_SIZE is
not overridden and users can control the recording size again.

Video/video.sh

@@ -71,7 +71,9 @@ function wait_for_display() {
   until xset b off >/dev/null 2>&1; do
     sleep ${poll_interval}
   done
-  VIDEO_SIZE=$(xdpyinfo | grep 'dimensions:' | awk '{print $2}')
+  if [ -z "$SE_SCREEN_WIDTH" -a -z "$SE_SCREEN_HEIGHT"]; then
+    VIDEO_SIZE=$(xdpyinfo | grep 'dimensions:' | awk '{print $2}')
+  fi
   echo "$(date -u +"${ts_format}") [${process_name}] - Display ${DISPLAY} is open with dimensions ${VIDEO_SIZE}"
 }