Change wxSize::Scale() to take double, not float

There doesn't seem to be any reason to use float here when double is
used everywhere else.

Moreover, the (implicit) conversion of int to float is not always
lossless, and resulted in -Wconversion warnings from g++ 7, which
disappear when using doubles.

interface/wx/gdicmn.h

@@ -1014,7 +1014,7 @@ public:
         @return A reference to this object (so that you can concatenate other
                  operations in the same line).
     */
-    wxSize& Scale(float xscale, float yscale);
+    wxSize& Scale(double xscale, double yscale);
 
     /**
         Sets the width and height members.