8357034: GifImageDecoder can produce wrong transparent pixels

[mickleness]

If there are two consecutive frames that use DISPOSAL_SAVE, but the transparent pixel index changed: we might accidentally send the wrong data to the ImageConsumer.

We already had logic that submits info "the hard way" (see comment in code); this PR just makes sure we trigger that block.

I wrote four PRs that share the GifComparison class in this PR. Once any of them clear code review the other PRs will be much simpler:

1. 8357034 (this one)
2. 8356137
3. 8356320
4. 8351913

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Progress

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Issue

• JDK-8357034: GifImageDecoder can produce wrong transparent pixels (Bug - P4)

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