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/oiio/oiiodecoder.h

@@ -57,6 +57,8 @@ public:
 
   static PixelFormat::Format GetFormatFromOIIOBasetype(const OIIO::ImageSpec& spec);
 
+  static rational GetPixelAspectRatioFromOIIO(const OIIO::ImageSpec& spec);
+
 private:
 #if OIIO_VERSION < 10903
   OIIO::ImageInput* image_;