Document wxMulDivInt32() and add a test for it

This function exists since always and is probably already used outside
wx, so make it officially public and add at least a trivial unit test
for it.

interface/wx/math.h

@@ -115,5 +115,15 @@ bool wxIsSameDouble(double x, double y);
 */
 bool wxIsNullDouble(double x);
 
+/**
+    Computes the product of a number with a fraction with rounding.
+
+    This function returns @c n*numerator/denominator rounding the result. It is
+    similar to the standard Win32 @c MulDiv() function and, in fact, is
+    implemented by calling it under MSW, where @c wx/msw/wrapwin.h must be
+    included in addition to @c wx/math.h for it to be used.
+ */
+int wxMulDivInt32(int n, int numerator, int denominator);
+
 //@}
 

tests/misc/misctests.cpp

@@ -22,6 +22,11 @@
 #include "wx/tarstrm.h"
 #include "wx/zipstrm.h"
 
+#ifdef __WINDOWS__
+    // Needed for wxMulDivInt32().
+    #include "wx/msw/wrapwin.h"
+#endif
+
 // ----------------------------------------------------------------------------
 // test class
 // ----------------------------------------------------------------------------
@@ -199,3 +204,12 @@ TEST_CASE("wxRound", "[math]")
     #endif
 #endif // WXWIN_COMPATIBILITY_3_0
 }
+
+TEST_CASE("wxMulDivInt32", "[math]")
+{
+    // Check that it rounds correctly.
+    CHECK( wxMulDivInt32(15, 3, 2) == 23 );
+
+    // Check that it doesn't overflow.
+    CHECK( wxMulDivInt32((INT_MAX - 1)/2, 200, 100) == INT_MAX - 1 );
+}