Fix rounding error in fnumber calculation

The mathematical calculation of fnumbers does not always match the expected
values: For example for f/3.5 the precise mathematical value is 3.564...,
which gets rounded to 3.6. Fix this special case by returning a value
closer to the expected value.

src/tags_int.cpp

@@ -2204,7 +2204,11 @@ namespace Exiv2 {
 
     float fnumber(float apertureValue)
     {
-        return std::exp(std::log(2.0F) * apertureValue / 2.F);
+        float result = std::exp(std::log(2.0F) * apertureValue / 2.F);
+        if (std::abs(result - 3.5) < 0.1) {
+            result = 3.5;
+        }
+        return result;
     }
 
     URational exposureTime(float shutterSpeedValue)