IMAGE: Fix cinepak conversion to 8bpp for some Myst (1994) videos

[antoniou79]

This addresses bug ticket #13479

The fix should not have side-effects, but someone more familiar with the code for the cinepak decoder should review this.

I can see some black artifacts at the start of the videos so maybe something else is needed as well, or a different approach altogether. Relevant short discord discussion starts here: https://discord.com/channels/581224060529148060/581224061091446795/975812864721956924

[sluicebox]

Is this the same bug that I'm fixing in QuickTimeDecoder? #4156

If so, then it's a regression I caused in November when I fixed some QuickTimeDecoder bugs but broke a rule I didn't know about where decoding a frame isn't allowed when loading a stream, because that in turn breaks VideoDecoder::setDitheringPalette

From the comments on VideoDecoder::setDitheringPalette:

* This should be called after loadStream(), but before a decodeNextFrame()
* call. This is enforced.

I don't have Myst but I would start by testing the QuickTimeDecoder fix and see if that takes care of this.

[antoniou79]

Is this the same bug that I'm fixing in QuickTimeDecoder? #4156

If so, then it's a regression I caused in November when I fixed some QuickTimeDecoder bugs but broke a rule I didn't know about where decoding a frame isn't allowed when loading a stream, because that in turn breaks VideoDecoder::setDitheringPalette

From the comments on VideoDecoder::setDitheringPalette:

* This should be called after loadStream(), but before a decodeNextFrame()
* call. This is enforced.

I don't have Myst but I would start by testing the QuickTimeDecoder fix and see if that takes care of this.

You're correct! Your code works, and it works better -- no black artifacts at the start of video. So this PR can be closed.

I actually was reminded that this Myst bug and proposed patch were pending for a while, because I read your PR today, but it didn't seem directly related at least not by looking at the files. But, I did intend to check it out anywyay, then other life things happened, so thanks for the extra nudge!

[sluicebox]

Thanks for testing that; I've updated my commit message to include Myst and the bug ticket