RFR: 8356320: GifImageDecoder can produce wrong image when GCE changes transparent pixel index [v2]

[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.
> 
> (I know this looks similar to https://github.com/openjdk/jdk/pull/25044 , but if you look at the data structure of the sample gifs: they are separate problems with separate solutions.)

Jeremy Wood has updated the pull request incrementally with five additional commits since the last revision:

 - 8356320: fixing copy & paste typo
 - GifComparison: fixing error message
 - GifComparison: code cleanup
 - 8356320: change package to access GifComparison
 - GifComparison: adding common helper class for gif bugs

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25076/files
  - new: https://git.openjdk.org/jdk/pull/25076/files/da529dcd..178c006f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25076&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25076&range=00-01

  Stats: 586 lines in 4 files changed: 447 ins; 139 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