rational: convert to int when returning AVRational

Fixes potential warnings, as AVRational only deals with int.

Change-Id: I35d53a1a84b9e7f9709a51a4c8be8873ba4f7094

src/rational.h

@@ -49,7 +49,7 @@ public:
 
     // Define conversions to and from AVRational (equivalent)
     rational(AVRational r) : num_(r.num), den_(r.den) {};
-    operator AVRational() const { return AVRational{num_, den_}; }
+    operator AVRational() const { return AVRational{(int)num_, (int)den_}; }
 
     // Normal copy constructors and assignment operators