Patch FFmpeg to allow preset -1 (MR)

Preset -1 is currently not allowed using ffmpeg given it's being capped

https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/libsvtav1.c#L173

Need to update the default preset value to -2 and include -1 before the assignment. We also need to check if pinging the preset from svt-params would allow -1.