VP9 decodes to scrambled video after fefe4b97

[wangqr]

Original issue: wangqr/Aegisub#32
Test video: https://youtu.be/MNcnT8wBcdw or https://ftp.wangqr.tk/tmp/%E5%BF%83%E5%81%9A%E3%81%97%20_%20%E5%A4%8F%E8%89%B2%E3%81%BE%E3%81%A4%E3%82%8A%20cover%E3%80%90%E3%82%AA%E3%83%AA%E3%82%B8%E3%83%8A%E3%83%ABPV_4K%E3%80%91-MNcnT8wBcdw.webm (both links are the same video, latter may be deleted shortly after)

This is a regression bug, which does not present in 2.23.1 release. git bisect shows that fefe4b9 is the first bad commit.

fefe4b979d3e04de8dc3383be4f85e3778d4ae6d is the first bad commit
commit fefe4b979d3e04de8dc3383be4f85e3778d4ae6d
Author: myrsloik <fredrik.mellbin@gmail.com>
Date:   Mon Mar 27 14:25:01 2017 +0200

    Add back original videosource hacks

 src/core/videosource.cpp | 32 ++++++++++++++++++++++++++------
 src/core/videosource.h   |  4 ++++
 2 files changed, 30 insertions(+), 6 deletions(-)

[ghost]

I confirm: 2.40 is unable to decode correctly 4K VP9 streams from youtube source. It gives black frames or scrambled frames as shown above, and is very slow. 2.23 was much better because it could decode, but it only skipped/added frames !
I only tested 4K VP9 but didn't test lower resolutions.

[myrsloik]

Appears to at least not have any obvious decoding errors with @dwbuiten's PR. Seeking is however not completely frame accurate for some reason.