lavu: add av_gcd_q().

libavutil/rational.h

@@ -207,6 +207,12 @@ int av_find_nearest_q_idx(AVRational q, const AVRational* q_list);
  */
 uint32_t av_q2intfloat(AVRational q);
 
+/**
+ * Return the best rational so that a and b are multiple of it.
+ * If the resulting denominator is larger than max_den, return def.
+ */
+AVRational av_gcd_q(AVRational a, AVRational b, int max_den, AVRational def);
+
 /**
  * @}
  */