RFR: 8356137: GifImageDecode can produce opaque image when disposal method changes [v7]

[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: remove the redundant transparency check
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25044#discussion_r2109318427
 - 8356137: flood fill saved_image at initialization
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25044#issuecomment-2912699099

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25044/files
  - new: https://git.openjdk.org/jdk/pull/25044/files/19804549..62c2bf0d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25044&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25044&range=05-06

  Stats: 20 lines in 2 files changed: 4 ins; 16 del; 0 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