viewer: implemented basic deinterlace

Implements a very basic GLSL deinterlace that simply halves the vertical
resolution and then interpolates between the fields. This can be toggled
on or off.

The reasons for being so basic is:
- Speed, very quick code running in OpenGL
- It would seem the highest quality deinterlacers are temporally based
  which doesn't make much sense for the viewer, particularly since we can't
  double the frame rate since our timecode is fixed to the frames.

Higher quality interlacing/deinterlacing will be present in the actual
renderer.

app/node/output/viewer/viewer.cpp

@@ -137,6 +137,7 @@ 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();
+  bool interlacing_changed = video_params_.interlacing() != video.interlacing();
 
   video_params_ = video;
 
@@ -148,6 +149,10 @@ void ViewerOutput::set_video_params(const VideoParams &video)
     emit PixelAspectChanged(video_params_.pixel_aspect_ratio());
   }
 
+  if (interlacing_changed) {
+    emit InterlacingChanged(video_params_.interlacing());
+  }
+
   if (timebase_changed) {
     video_frame_cache_.SetTimebase(video_params_.time_base());
     emit TimebaseChanged(video_params_.time_base());

app/node/output/viewer/viewer.h

@@ -133,6 +133,8 @@ signals:
 
   void PixelAspectChanged(const rational& pixel_aspect);
 
+  void InterlacingChanged(VideoParams::Interlacing mode);
+
   void VideoParamsChanged();
   void AudioParamsChanged();