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/codec/frame.h

@@ -57,9 +57,6 @@ public:
   bool contains_pixel(int x, int y) const;
   void set_pixel(int x, int y, const Color& c);
 
-  const rational& sample_aspect_ratio() const;
-  void set_sample_aspect_ratio(const rational& sample_aspect_ratio);
-
   /**
    * @brief Get frame's timestamp.
    *
@@ -116,8 +113,6 @@ private:
 
   int64_t native_timestamp_;
 
-  rational sample_aspect_ratio_;
-
   int linesize_;
 
 };