Only set GIF canvas background when required.

[JimBobSquarePants]

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Description

Fix #2980

[Copilot]

Pull Request Overview

This PR fixes issue #2980 by improving how GIF decoder and encoder handle canvas background colors, particularly ensuring that background colors are only set when required and that transparency handling is correctly applied.

• Optimizes background color handling to only apply when disposal methods require it
• Removes unnecessary transparency alpha manipulation during decoding
• Adds proper background index calculation that respects metadata values

Reviewed Changes

Copilot reviewed 74 out of 74 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/Images/Input/Gif/issues/issue_2980.gif	Adds test GIF file for issue reproduction
tests/Images/External/ReferenceOutput/GifDecoderTests/Issue2980_*	New reference output frames for issue 2980 test
tests/Images/External/ReferenceOutput/GifDecoderTests/*	Updated reference outputs reflecting improved transparency handling
tests/ImageSharp.Tests/TestImages.cs	Adds constant for new test GIF
tests/ImageSharp.Tests/Quantization/QuantizedImageTests.cs	Removes transparency assertions that are no longer valid
tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs	Adds test case for issue 2980
src/ImageSharp/Formats/Gif/GifEncoderCore.cs	Improves background index calculation and transparency handling
src/ImageSharp/Formats/Gif/GifDecoderCore.cs	Optimizes canvas background setting logic

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[Copilot]

The method returns ClampIndex(match) but match could be -1, which would be clamped to 0. This could incorrectly return index 0 as the background when no match is found and no valid background index exists from metadata.

Suggested change

	return ClampIndex(match);
	// If no match was found, return byte.MaxValue to indicate "no valid background index".
	return match == -1 ? byte.MaxValue : ClampIndex(match);

[JimBobSquarePants]

0 is the default in GIF