make an improvement about transparency of the preview window, thanks!

[zp2546265641]

When we start recording, on the main page, we can preview the window! Great!
But there is a small problem, the main window can also be seen in the preview window, I hope we can adjust the transparency of the main window. if so, we can do recording at one hand, at the other hand, we can see the effection of the recording.Thanks!