RFR: 8356320: GifImageDecoder can produce wrong image when GCE changes transparent pixel index [v3]

[Jeremy Wood]

> When a gif frame's disposal code is DISPOSAL_BGCOLOR and we're maintaining a saved image: we have to flood-fill the frame's bounds. And if the transparent pixel index changes across frames: we could sometimes flood fill the wrong value, resulting in an opaque rectangle in the output image.
> 
> There are a cluster of four related PRs that share the GifComparison class in this PR.
> 
> 1. [8357034](https://github.com/openjdk/jdk/pull/25264)
> 2. ~~[8356137](https://github.com/openjdk/jdk/pull/25044)~~ (integrated)
> 3. [8356320](https://github.com/openjdk/jdk/pull/25076) (this one)
> 4. [8351913](https://github.com/openjdk/jdk/pull/24271)
> 
> The test in this PR required [8356137's PR](https://github.com/openjdk/jdk/pull/25044) to pass.
> 
> This bug can be observed reading this gif animation:
> https://tenor.com/view/fnf-free-download-fnf-free-download-fnf-fd-cyan-gif-15334412416964826224

Jeremy Wood has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 31 commits:

 - Merge branch 'master' into JDK-8356320
   
   # Conflicts:
   #	test/jdk/sun/awt/image/gif/GifBuilder.java
   #	test/jdk/sun/awt/image/gif/GifComparison.java
 - Merge branch 'master' into JDK-8356320
 - 8356320: Use new GifBuilder and remove leo.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
 - 8356137: Adding copyright to GifComparison
 - 8356320: trivial whitespace and comment changes
 - Merge branch 'master' into JDK-8356320
 - ... and 21 more: https://git.openjdk.org/jdk/compare/a86dd56d...3deeb497

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

Changes: https://git.openjdk.org/jdk/pull/25076/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25076&range=02
  Stats: 40 lines in 2 files changed: 40 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25076/head:pull/25076

PR: https://git.openjdk.org/jdk/pull/25076