various: allow setting footage aspect ratios

We had support for detecting aspect ratios and respecting them in the
render, but this allows people to not only see the aspect ratio in use,
but also override it with their own.

app/codec/ffmpeg/ffmpegdecoder.h

@@ -64,8 +64,15 @@ public:
   void RemoveFramesBefore(const qint64& t);
   void TruncateCacheRangeTo(const qint64& t);
 
-  rational sample_aspect_ratio() const;
-  AVStream* stream() const;
+  AVFormatContext* fmt_ctx() const
+  {
+    return fmt_ctx_;
+  }
+
+  AVStream* stream() const
+  {
+    return avstream_;
+  }
 
   void ClearFrameCache();
 
@@ -191,7 +198,6 @@ private:
   PixelFormat::Format native_pix_fmt_;
 
   rational time_base_;
-  rational aspect_ratio_;
   int64_t start_time_;
 
   static QHash< Stream*, QList<FFmpegDecoderInstance*> > instance_map_;