Upon starting screen curtain, Automatically perform test that screen curtain is operating.

[derekriemer]

Is your feature request related to a problem? Please describe.

It is theoretically possible for the translucent overlay used to cause NVDA's screen curtain to fail.
As seen in the 2021.2 whats new:

Due to a change in the Windows Magnification API, Screen Curtain had to be updated to support the newest versions of Windows. Use NVDA 2021.2 to activate Screen Curtain with Windows 10 21H2 (10.0.19044) or later. This includes Windows 10 Insiders and Windows 11. For security purposes, when using a new version of Windows, get visual confirmation that the Screen Curtain makes the screen entirely black.

Describe the solution you'd like

Upon the screen curtain activating, schedule a timer for some reasonable amount of time, perhaps 1 second.
Upon this timer firing, perform an OCR of the screen, and extract the text. If our translucent overlay is peroperly opperating, the text that is detected will be empty. If not, perform the ocr check a few more times on shorter, say .5 second intervals, and if text is still appearing, pop a dialog alerting the user that screen curtain is broken.

[lukaszgo1]

Duplicate of #12708

[seanbudd]

Closing as this is a duplicate of #12708