vp8 VBR encoding targets to fixed 256kbit/s rate

[Bastiaan Welmers]

Since version 5.0 the vp8 encoding when only setting -crf and setting -b:v 0 is suddenly targeted to 256kbit/s. This wasn't the case in previously installed version 4.1. Needless to say, 256kbit/s results in very poor quality videos.
I assume this to be a bug, because Variable Bit Rate encodings for libvpx-vp8 never needed any targeting bitrate, only a CRF value, the bitrate was always advised to set to 0 and it works fine like that. I also cannot find a way to unset the 256kbit/s target for VBR mode, removing the `-b:v 0' option does not help.

How to reproduce:

% ffmpeg -i $in -acodec libvorbis -ac 2 -b:a 96k -vcodec libvpx -b:v 0 -crf 33 -threads 0 -s 854x480 $base.480p-vp8.webm
ffmpeg version 5.0.1 and git-20220302

In the output, the following message appears, not apearing in 4.1 where the bug is not present:

[libvpx @ 0x6b6b780] Bitrate not specified for constrained quality mode, using default of 256kbit/sec

Perhaps this bug is introduced here:
14941d386acd90b92aaaf75fbdac2c07d33f167a

[Carl Eugen Hoyos]

Please test current FFmpeg git head and provide the command line you tested together with the complete, uncut console output to make this a valid ticket.