RFR: 8357034: GifImageDecoder can produce wrong transparent pixels [v3]

[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.
> 
> 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](https://github.com/openjdk/jdk/pull/25264) (this one)
> 2. [8356137](https://github.com/openjdk/jdk/pull/25044)
> 3. [8356320](https://github.com/openjdk/jdk/pull/25076)
> 4. [8351913](https://github.com/openjdk/jdk/pull/24271)

Jeremy Wood has updated the pull request incrementally with one additional commit since the last revision:

  8356137: Adding copyright to GifComparison

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25264/files
  - new: https://git.openjdk.org/jdk/pull/25264/files/b1a1d7ec..3d94c434

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=01-02

  Stats: 22 lines in 1 file changed: 22 ins; 0 del; 0 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