Windows/FFmpeg: import multi-plane textures directly on D3D12/OpenGL

isPixelFormatSupportedByRhiBackend() now accepts NV12/P010 on D3D12
and OpenGL too, not just single-plane formats: importTexture()
splits the source texture via QWindowsD3D11PlaneSplitter, imports
each plane through the existing single-plane path, and combines the
results into one QVideoFrameTexturesHandles.

With that gate widened, the eager pixel-format override in
QFFmpeg::VideoRenderer and QFFmpegVideoBuffer is no longer needed
and is removed, so a frame now always declares its real decoded
pixel format.

Task-number: QTBUG-115309
Change-Id: I297cd4ae9095d3239f9e1678f21d863fe9b17d84
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>