Use the current video aspect ratio if the video stream is reset.

In AvFormatDecoder, if a stream change is detected and no
new/replacement video stream is found, we choose some sensible defaults
and tell the player about the change. The video aspect ratio is however
reset to the default 4/3 - which immediately changes the currently
displayed frames. Instead use a pre-existing aspect ratio, which will be
4/3 by default if we haven't yet determined something more sensible.

This is only likely to be an issue with DVD playback, where we often get
stream updates piecemeal from the demuxer - the streams are reset and
some new audio streams added (no video found) and the new video stream
is added a 'short' period of time later.

Refs #9854

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -2155,12 +2155,12 @@ int AvFormatDecoder::ScanStreams(bool novideo)
             tvformat == "pal-m" || tvformat == "atsc")
         {
             fps = 29.97;
-            m_parent->SetVideoParams(-1, -1, 29.97, 1);
+            m_parent->SetVideoParams(-1, -1, 29.97, 1, current_aspect);
         }
         else
         {
             fps = 25.0;
-            m_parent->SetVideoParams(-1, -1, 25.0, 1);
+            m_parent->SetVideoParams(-1, -1, 25.0, 1, current_aspect);
         }
     }