DXDIAG: Fullscreen DirectDraw test leaves taskbar visible

Steps to reproduce:

• Start ReactOS Live CD
• Press Win+R, type dxdiag and hit Enter
• Go to "Display 1" page and press "Test DirectDraw"
• Press buttons in a sequence: Yes, No, No, Yes (this will run only 3rd fullscreen test)
• Expected: The test runs in fullscreen
• Observed: The taskbar is visible on top of test screen

Tested and confirmed on build 0.4.15-dev-5114-g7278ed2. Replacing ddraw.dll with one from Windows Server 2003 fixes the problem - taskbar disappears after a second from the test start, however the white square is not visible, only the black screen.

PR #4728 does fix the problem (taskbar disappears immediately), however it cannot be merged because it was authored by Bryan Kirk. We need somebody from the team to commit the optimal fix. Since this happens with an out-of-box application, it should be easier to analyze it.

Debug log: 0.4.15-dev-5114-g7278ed2.txt