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.

include/wx/gdicmn.h

@@ -318,7 +318,7 @@ public:
     void DecBy(int d) { DecBy(d, d); }
 
 
-    wxSize& Scale(float xscale, float yscale)
+    wxSize& Scale(double xscale, double yscale)
         { x = (int)(x*xscale); y = (int)(y*yscale); return *this; }
 
     // accessors