FFmpeg 9.1 Adds Vulkan Encoder And NVIDIA Support

FFmpeg 9.1 prepares for release featuring a new APV Vulkan encoder and an NVIDIA Optical Flow accelerated filter named vffrucvulkan. The update also includes extensively improved AAC encoder performance, ASTC image encoding support via libastcenc, and D3D12 DirectML zero-copy inference for the ONNX Runtime DNN backend alongside various other new decoders and filters.