• Use ffmpeg for svt-av1 encodes instead of invoking to SvtAv1EncApp directly. This unifies the handling of
  other encoders & allows svt-av1 encoding to benefit from more built-in ffmpeg behaviours like aspect preservation.
  An ffmpeg build with libsvtav1 enabled is now required. SvtAv1EncApp is no longer required.
• Improve image detection.
• Add --encoder support for nvenc family of ffmpeg encoders: av1_nvenc, hevc_nvenc, and h264_nvenc.