RFR: 8357034: GifImageDecoder can produce wrong transparent pixels [v4]

[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 five additional commits since the last revision:

 - 8356320: Use new GifBuilder and remove ukraine-flag.gif
   
   This is an extension of work for this PR:
   https://github.com/openjdk/jdk/pull/25044#pullrequestreview-2871107750
 - 8356137: adding copyright
 - 8356137: Adding GifBuilder to dynamically create test file
   
   This can be used by multiple gif tests in this directory.
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25044#pullrequestreview-2871107750
 - 8356137: trivial javadoc update
 - 8356137: only inspect last frame of gif
   
   This makes the main() method much less useful, so I removed it too. (I originally used this class to explore a folder of hundreds of gifs to look for discrepancies. But the discrepancies were rarely only on the last frame.)
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25044#discussion_r2109298270

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25264/files
  - new: https://git.openjdk.org/jdk/pull/25264/files/3d94c434..3750e58e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=02-03

  Stats: 262 lines in 4 files changed: 152 ins; 67 del; 43 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