RFR: 8356137: GifImageDecode can produce opaque image when disposal method changes [v8]

[Jeremy Wood]

> This resolves a gif parsing bug where an unwanted opaque rectangle could appear under these conditions:
> 
> 1. The disposal method for frames is 1 (meaning "do not dispose", aka "DISPOSAL_SAVE")
> 2. The transparent pixel is non-zero
> 3. There's more than one such consecutive frame
> 
> Previously: the GifImageDecoder would leave the saved_image pixels as zero when they were supposed to be transparent. This works great if the transparent pixel index is zero, but it flood fills the background of your frame with the zeroeth color otherwise.
> 
> 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)
> 2. [8356137](https://github.com/openjdk/jdk/pull/25044) (this one)
> 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 two additional commits since the last revision:

 - 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/25044/files
  - new: https://git.openjdk.org/jdk/pull/25044/files/62c2bf0d..e0546b1a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25044&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25044&range=06-07

  Stats: 92 lines in 1 file changed: 0 ins; 53 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/25044.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25044/head:pull/25044

PR: https://git.openjdk.org/jdk/pull/25044