Boost-1.65: fix another integer ambiguity problem with boost::rational

<rant>
the "improved" boost::rational can no longer compute 1/x
quite brilliant
</rant>

well... the reason is again signed vs unsigned int.
FrameRate is based on unsigned int (since a negative frame rate makes no sense).

src/lib/time/timevalue.hpp

@@ -572,6 +572,13 @@ namespace time {
       operator std::string() const;
     };
   
+  /** convenient conversion to duration in fractional seconds */
+  inline FSecs
+  operator/ (int n, FrameRate rate)
+  {
+    return n / boost::rational_cast<FSecs> (rate);
+  }
+