lavu/rational: add syntactic sugar.

Add a function to create a rational
and macros for common values.

libavutil/rational.h

@@ -45,6 +45,17 @@ typedef struct AVRational{
     int den; ///< denominator
 } AVRational;
 
+/**
+ * Create a rational.
+ * Useful for compilers that do not support compound literals.
+ * @note  The return value is not reduced.
+ */
+static inline AVRational av_make_q(int num, int den)
+{
+    AVRational r = { num, den };
+    return r;
+}
+
 /**
  * Compare two rationals.
  * @param a first rational