win32: do not erase background

src/win32/fresnel.win32.pas

@@ -424,12 +424,12 @@ end;
 
 procedure TWin32WSForm.Invalidate;
 begin
-  Windows.InvalidateRect(FWindow,nil,true);
+  Windows.InvalidateRect(FWindow,nil,false);
 end;
 
 procedure TWin32WSForm.InvalidateRect(const aRect: TFresnelRect);
 begin
-  Windows.InvalidateRect(FWindow,aRect.GetRect,true);
+  Windows.InvalidateRect(FWindow,aRect.GetRect,false);
 end;
 
 procedure TWin32WSForm.HandleMouseMsg(XPos, YPos: longint;