RFR: 8357034: GifImageDecoder can produce wrong transparent pixels

[Jeremy Wood]

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.

There are 4 tickets/PRs that use this new GifComparison class. I recommend not reviewing this PR until #25044 clears code review. (See https://github.com/openjdk/jdk/pull/25044#issuecomment-2881889090 )

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Commit messages:
 - 8357034: fixing ticket number
 - GifComparison: fixing error message
 - Incident 9078482: fix when new trans index is in saved images
 - GifComparison: code cleanup
 - GifComparison: adding common helper class for gif bugs
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'openjdk:master' into master
 - ... and 7 more: https://git.openjdk.org/jdk/compare/5e50a584...5b982b12

Changes: https://git.openjdk.org/jdk/pull/25264/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357034
  Stats: 471 lines in 4 files changed: 470 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25264.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25264/head:pull/25264

PR: https://git.openjdk.org/jdk/pull/25264