RFR: 8357034: GifImageDecoder can produce wrong transparent pixels [v8]

[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.
> 
> 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) (this one)
> 2. ~~[8356137](https://github.com/openjdk/jdk/pull/25044)~~ (integrated)
> 3. [8356320](https://github.com/openjdk/jdk/pull/25076)
> 4. [8351913](https://github.com/openjdk/jdk/pull/24271)
> 
> This bug can be observed reading these gif animations:
> 
> https://giphy.com/gifs/fomoduck-duck-fomo-ZUlzR40oGACqNmy0q8
> https://media4.giphy.com/media/Zbf4JQzcDhzeraQvk9/giphy.gif
> https://giphy.com/gifs/computer-working-cat-LHZyixOnHwDDy
> https://giphy.com/gifs/90s-fgfgif-r4BmI9xUaDuKJQdd3l
> 
> ### Describing This Bug
> 
> The steps/explanation that lead to this failure are a little complicated. I'll try to summarize the original bug here.
> 
> All three frames in the unit test's GIF use disposal code doNotDispose, meaning the frames are layered one on top of the previous.
> 
> The first frame produces 3 circles from left to right using these colors:
> 0xff0000 (red) 0xffff00 (yellow) 0x00ff00 (green)
> 
> The background is 0x00ffff (cyan). It is the transparent pixel, so the first frame is:
> 
> <img width="200" height="100" alt="frame_0" src="https://github.com/user-attachments/assets/8bb53530-fc2b-4a8b-b3a6-5c58782ea5ab" />
> 
> Now we start processing the second frame. The pixels are the same, but the color model has changed so the zeroeth color (red) is the transparent index.
> 
> When the GifImageDecoder layers the 2nd frame on top of the 1st frame, it notices that `model.equals(saved_model)` is `false`. This means it enter a block of code that refers to "the hard way" of passing new pixel information to the ImageConsumers.
> 
> The 2nd frame, after being painted on the 1st frame, looks like:
> <img width="200" height="100" alt="frame_1" src="https://github.com/user-attachments/assets/0c8ca55b-e25d-418e-bb95-2e6f9e427c08" />
> 
> The 3rd frame is exactly the same as the 2nd. But here's where the bug is: now `model.equals(saved_model)` is `true`, so GifImageDecoder tries to pass the pixel information the easy way. The result is:
> 
> <img width="200" height="100" alt="frame_2_awt" src="https://github.com/user-attachments/assets/835dd918-f1bc-459e-9967-295fd01fa61c" />
> 
> Because it'...

Jeremy Wood has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 35 commits:

 - Merge branch 'master' into JDK-8357034
 - 8357034: switch to `isSavedModelReliable`
   
   This reduces how often we resort to "the hard way". The previous implementation would resort to the hard way *any* time the disposal code was SAVE and there was a transparent pixel in use.
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25264/files#r2292318270
 - Merge branch 'master' into JDK-8357034
 - 8357034: re-wrapping line breaks
 - 8356137, 8357034: add debug option to dump PNGs of frames
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25044#pullrequestreview-3007487527
 - Merge branch 'master' into JDK-8357034
   
   # Conflicts:
   #	test/jdk/sun/awt/image/gif/GifBuilder.java
   #	test/jdk/sun/awt/image/gif/GifComparison.java
 - 8357034: avoid recalculating isSimpleSavedImageComparison for every line
   
   This is in response to:
   https://github.com/openjdk/jdk/pull/25264#discussion_r2197097522
 - Merge branch 'master' into JDK-8357034
 - 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
 - ... and 25 more: https://git.openjdk.org/jdk/compare/1781b186...ece8707a

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

Changes: https://git.openjdk.org/jdk/pull/25264/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25264&range=07
  Stats: 147 lines in 5 files changed: 112 ins; 4 del; 31 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