V3 : Fix GIF and WEBP Edge Case Handling

[JimBobSquarePants]

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Description

Fixes #2866

There's quite a lot going on here:

• Both decoders we not handling frame disposal properly.
• GIF Decoder background color handling was incorrect.
• GIF Encoder incorrectly used global palette for local root frame.
• WEBP Decoder did not clear some buffers on load where it should.

I'll have to manually rework this for V4 as the code base has migrated significantly there.

Copilot reviewed 5 out of 16 changed files in this pull request and generated no comments.

Files not reviewed (11)

• tests/Images/External/ReferenceOutput/GifDecoderTests/Issue1962_Rgba32_issue1962_tiniest_gif_1st.png: Language not supported
• tests/Images/External/ReferenceOutput/GifDecoderTests/Issue2012BadMinCode_Rgba32_issue2012_drona1.png: Language not supported
• tests/Images/Input/Gif/issues/issue_2866.gif: Language not supported
• tests/Images/Input/Webp/issues/Issue2866.webp: Language not supported
• src/ImageSharp/Formats/Gif/GifEncoderCore.cs: Evaluated as low risk
• tests/ImageSharp.Tests/TestUtilities/ReferenceCodecs/MagickReferenceDecoder.cs: Evaluated as low risk
• src/ImageSharp/Formats/Webp/Lossy/Vp8Decoder.cs: Evaluated as low risk
• src/ImageSharp/Formats/Webp/BitReader/BitReaderBase.cs: Evaluated as low risk
• tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs: Evaluated as low risk
• tests/ImageSharp.Tests/TestImages.cs: Evaluated as low risk
• src/ImageSharp/Formats/Webp/WebpBlendMethod.cs: Evaluated as low risk

Comments suppressed due to low confidence (2)

tests/ImageSharp.Tests/Formats/Gif/GifEncoderTests.cs:389

• [nitpick] The test method name GifEncoder_CanDecode_Issue2866 is misleading since it is testing encoding, not decoding. It should be renamed to GifEncoder_CanEncode_Issue2866.

public void GifEncoder_CanDecode_Issue2866<TPixel>(TestImageProvider<TPixel> provider)

src/ImageSharp/Formats/Webp/WebpAnimationDecoder.cs:89

• The comment should reflect the correct type for backgroundColor, which is TPixel instead of Color.

/// <param name="backgroundColor">The default background color of the canvas in.</param>