various: implement sequence pixel aspect ratios and interlacing settings

Implements the following:
- Sequences have pixel aspect ratios that work in tandem with footage PARs
  to render footage correctly. Viewer and export also acknowledge PARs
- Sequences can have interlacing settings. This doesn't do anything yet,
  eventually the renderer will need to interlace/deinterlace/reinterlace
  appropriately in order to conform all the footage to the sequence. Export
  acknowledges interlacing, but this only affects metadata, not the image.

app/node/output/viewer/viewer.cpp

@@ -136,6 +136,7 @@ void ViewerOutput::set_video_params(const VideoParams &video)
 {
   bool size_changed = video_params_.width() != video.width() || video_params_.height() != video.height();
   bool timebase_changed = video_params_.time_base() != video.time_base();
+  bool pixel_aspect_changed = video_params_.pixel_aspect_ratio() != video.pixel_aspect_ratio();
 
   video_params_ = video;
 
@@ -143,6 +144,10 @@ void ViewerOutput::set_video_params(const VideoParams &video)
     emit SizeChanged(video_params_.width(), video_params_.height());
   }
 
+  if (pixel_aspect_changed) {
+    emit PixelAspectChanged(video_params_.pixel_aspect_ratio());
+  }
+
   if (timebase_changed) {
     video_frame_cache_.SetTimebase(video_params_.time_base());
     emit TimebaseChanged(video_params_.time_base());

app/node/output/viewer/viewer.h

@@ -131,6 +131,8 @@ signals:
 
   void SizeChanged(int width, int height);
 
+  void PixelAspectChanged(const rational& pixel_aspect);
+
   void VideoParamsChanged();
   void AudioParamsChanged();