V4 : Fix GIF, PNG, and WEBP Edge Case Handling

[JimBobSquarePants]

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Description

#2882 but for V4.

Fixes #2866
Fixes #2862

Changes shared with V3:

• Both GIF and WEBP decoders were 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.
• PNG Encoder palette animations did not work properly
• Fixed pixel sampling in Wu and Octree quantizers
• Improved accuracy for matching first 512 colors in EuclidianPixelMap.

V4 Specific changes

• Rewrite the OctreeQuantizer to support 32bit colors and added memory pooling to massively reduce memory.
• Added Alpha thresholding to quantizers.
• Replace Coarse caching with a dedicated type that uses 1/8th of the memory.
• Removes any decoded color tables should processing occur to allow generation of new tables on encode.

[JimBobSquarePants]

@rickbrew @saucecontrol This is the new "Octree" I've been chatting to you about.

[JimBobSquarePants]

@rickbrew @saucecontrol Here's the new Exact and Coarse caches I built. They work really well!

Copilot reviewed 177 out of 177 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/ImageSharp/Formats/Png/PngEncoderCore.cs:1482

• Ensure that 'backgroundColor' is reliably assigned before any use, as the [MemberNotNull] attribute enforces non-nullability and a missing assignment could lead to runtime violations.

[MemberNotNull(nameof(backgroundColor))]

src/ImageSharp/Formats/Png/PngEncoderCore.cs:1557

• Verify that 'paletteQuantizer' is defined as a nullable value type; if it is or becomes a reference type, using 'HasValue' may introduce nullability issues in future changes.

if (paletteQuantizer.HasValue)