Add APNG dispose and blend reference tests

[kesmeey]

Closes #3052.

This adds reference tests for all APNG dispose/blend operation combinations using the existing reference image infrastructure. No test harness or decoder code is changed.

Four APNG inputs are copied byte-for-byte from Web Platform Tests and renamed after the behavior they exercise:

• support/013.png -> fcTL-dispose-in-region-none.png
• support/015.png -> fcTL-dispose-in-region-background.png
• support/016.png -> fcTL-dispose-in-region-previous.png
• support/018.png -> fcTL-blend-source-transparent.png

WPT does not currently include APNG files for the background+source and previous+source combinations, so this also adds two minimal derived APNGs for those missing cases:

• dispose-background-blend-source.png
• dispose-previous-blend-source.png

The reference frames were generated with APNG Disassembler 2.9 (apngdis), independently of this crate's PNG decoder.

WPT attribution is included in tests/images/png/apng/wpt/README.txt, and the WPT BSD-3-Clause license is included in tests/images/png/apng/wpt/LICENSE.md.

I used AI assistance while creating and reviewing these test assets.

[RunDevelopment]

Thanks for working on this. This looks much better. Just one thing:

The APNG files are minimal 3x1 images derived from the Web Platform Tests APNG dispose/blend cases.

Is there a reason why you did this?

Unfortunately, this makes it harder to review and verify correctness. Had you copied the WPT files as is, I could just look at the decoded frames and use WPT as the reference for what they should be. Now I have to analyze the APNGs you added to see whether they actually have the properties they should. And if they do, I can't use WPT as a reference to check whether our decoder actually decodes them correctly.

So unless there's a good reason for using 3x1 images, could you please use the WPT APNGs as is?

[kesmeey]

Thanks for working on this. This looks much better. Just one thing:

The APNG files are minimal 3x1 images derived from the Web Platform Tests APNG dispose/blend cases.

Is there a reason why you did this?

Unfortunately, this makes it harder to review and verify correctness. Had you copied the WPT files as is, I could just look at the decoded frames and use WPT as the reference for what they should be. Now I have to analyze the APNGs you added to see whether they actually have the properties they should. And if they do, I can't use WPT as a reference to check whether our decoder actually decodes them correctly.

So unless there's a good reason for using 3x1 images, could you please use the WPT APNGs as is?

Thanks, that makes sense.

I updated the PR to copy the available WPT APNG files byte-for-byte and only rename them:

• support/013.png -> fcTL-dispose-in-region-none.png
• support/015.png -> fcTL-dispose-in-region-background.png
• support/016.png -> fcTL-dispose-in-region-previous.png
• support/018.png -> fcTL-blend-source-transparent.png

WPT does not seem to currently have APNG files for the background+source and previous+source combinations, so
those two are still derived minimal APNGs:

• dispose-background-blend-source.png
• dispose-previous-blend-source.png

I also updated the PR description/README to make that distinction clear.

[RunDevelopment]

I verified that the new images are indeed what the Readme.txt says they are, and everything checks out. Thank you!