fix screenshot generation for .webm with sparse keyframe

[SandiyosDev]

title

close #6499

ref

[Gykes]

No testing was done on this but won't this slow down cover/sprite/phash generation? We might be better off going some kind of try/catch situation where we try to do it the fast way, if it fails due to being to short, go the slower way.

[feederbox826]

Can you add phash parity testing before and after this change? If we can't ensure parity between this and previous versions then we have no choice but to reject

[SandiyosDev]

how about we try fast seek first and fall back to accurate seek on failure, I can add a ScreenshotTimeAccurate func and fallback at each call site
its the same idea as the existing SlowSeek pattern in the sprite generator

[feederbox826]

if parity tests fail theres no point

[SandiyosDev]

if I implement a fallback, it'll only trigger when fast seek fails entirely, so theres no parity chagne for files that already worked, they still use fast seek and get the same frame. only files that were previously erroring out would get a frame via accurate seek

[feederbox826]

then a parity test should be no issue at all