Backport of "d3d11videosink: Fix deadlock when parent window is busy"
Backport of
- "d3d11videosink: Fix deadlock when parent window is busy": !3570 (merged)
- "d3d11videosink: Protect window with lock at every place": 4e75ca03 in !2708 (merged)
- "d3d11videosink: Error out if RTV is unavailable": partial backport of the commit 5eeec165 in !2697 (merged)