AllowsTransparency = true会造成WindowsFormsHost消失

AllowsTransparency使WindowsFormsHost透明，从而无法看到嵌入的c++控件效果。

 

参考：

http://social.msdn.microsoft.com/Forums/vstudio/en-US/6f9dd3b5-af92-4076-9b4e-1a770dd52f70/windowsformshost-and-allowstransparency-makes-all-win32-controls-transparent?forum=wpf

http://stackoverflow.com/questions/4108531/wpf-windowsformshost-is-not-visible-when-allowstransparency-true

http://blog.csdn.net/hikaliv/article/details/5922871