Add wxWindow::MSWGetBorderThickness()

This function is more convenient to use in MSW-specific code than
GetWindowBorderSize() as it returns the size of a single border and so
doesn't require the caller to divide the return value by 2.

No real changes.

See #26571.

include/wx/msw/window.h

@@ -794,6 +794,8 @@ private:
     bool MSWSafeIsDialogMessage(WXMSG* msg);
 #endif // __WXUNIVERSAL__
 
+    int MSWGetBorderThickness() const;
+
     static inline bool MSWIsPositionDirectlySupported(int x, int y)
     {
         // The supported coordinate intervals for various functions are:

src/msw/window.cpp

@@ -2274,23 +2274,20 @@ void wxWindowMSW::DoSetClientSize(int width, int height)
     }
 }
 
-wxSize wxWindowMSW::GetWindowBorderSize() const
+int wxWindowMSW::MSWGetBorderThickness() const
 {
-    wxCoord border;
     switch ( GetBorder() )
     {
         case wxBORDER_STATIC:
         case wxBORDER_SIMPLE:
-            border = 1;
-            break;
+            return 1;
 
         case wxBORDER_SUNKEN:
         case wxBORDER_THEME:
-            border = 2;
-            break;
+            return 2;
 
         case wxBORDER_RAISED:
-            border = 3;
+            return 3;
             break;
 
         default:
@@ -2298,9 +2295,13 @@ wxSize wxWindowMSW::GetWindowBorderSize() const
             wxFALLTHROUGH;
 
         case wxBORDER_NONE:
-            border = 0;
+            return 0;
     }
+}
 
+wxSize wxWindowMSW::GetWindowBorderSize() const
+{
+    const auto border = MSWGetBorderThickness();
     return 2*wxSize(border, border);
 }