FFmpeg: fall back to QWindowsD3D11VideoProcessor on D3D12/OpenGL

D3D11TextureConverter now converts NV12 D3D11VA frames to BGRA8 via
QWindowsD3D11VideoProcessor before importing them, when the sink's
QRhi can't take the frame's pixel format directly (D3D12 and OpenGL
can only import single-plane textures). VideoRenderer decides this
once per frame, before the QVideoFrame is built, since its
QVideoFrameFormat is fixed for the frame's lifetime;
QFFmpegVideoBuffer gained a pixel-format override for this, which
only affects texture import, not the underlying hw frame or its CPU
map() fallback. qEnsureThreadLocalRhi() also gained a QRhi::D3D12
fallback.

Full window-based D3D12/OpenGL playback was not verified end-to-end,
as this environment has no reliable way to observe the sink's QRhi
actually getting attached.

Change-Id: I2579482fa541d7fd99b67c81fa8f6e404d357acb
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>