fix custom video mode with integer fps

It was creating a MLT profile with very large values in the
frame_rate_num and frame_rate_den.

https://forum.shotcut.org/t/beta-version-24-08-now-available-to-test/
45307/38

src/util.cpp

@@ -587,6 +587,9 @@ void Util::normalizeFrameRate(double fps, int &numerator, int &denominator)
         // Workaround storing QDoubleSpinBox::value() loses precision.
         numerator = qRound(fps * 1000000.0);
         denominator = 1000000;
+        auto gcd = greatestCommonDivisor(numerator, denominator);
+        numerator /= gcd;
+        denominator /= gcd;
     }
 }