Fix nonmonotonic DTS when stitching vfr content

Calculate i_bframe_delay_time from the maximum framerate for the level instead of the observed framerate when b_stitchable and b_vfr_input params are enabled. This prevents nonmonotonic DTS values when stitching high frame rate vfr too low frame rate vfr.

fatal: unable to access 'https://git.ffmpeg.org/ffmpeg.git/': server certificate verification failed. CAfile: none CRLfile: none

Happenes due to LetsEncrypt's DST Root CA X3 expiration.

FFmpeg devs still did not remove it from their chain and due to a bug in openssl...

https://www.hardenize.com/report/git.ffmpeg.org/1644186341#www_certs