V3 Backport : Reduce the number of memory allocations in lossless WebP encoder

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Backport of #2940 for V3.

[Copilot]

Pull Request Overview

This PR backports performance improvements for the lossless WebP encoder by reducing memory allocations and optimizing backward reference handling. Key changes include:

• Updating test and production code to use the new MemoryAllocator-based constructors for Vp8LBackwardRefs.
• Converting PixOrCopy from a class to a readonly struct and updating its creation methods.
• Refactoring loops to use foreach and in/ref semantics, as well as streamlining histogram and backward reference operations.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ImageSharp.Tests/Formats/WebP/Vp8LHistogramTests.cs	Updated instantiation of Vp8LBackwardRefs and literal token creation.
src/ImageSharp/Formats/Webp/Lossless/Vp8LHistogram.cs	Changed iteration in StoreRefs and updated AddSinglePixOrCopy to use in parameters.
src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	Removed block size calculation and updated references disposal.
src/ImageSharp/Formats/Webp/Lossless/Vp8LBackwardRefs.cs	Migrated from List to MemoryAllocator-based storage and added a Clear method.
src/ImageSharp/Formats/Webp/Lossless/PixOrCopy.cs	Converted to a readonly struct with updated factory methods for immutability.
src/ImageSharp/Formats/Webp/Lossless/HistogramEncoder.cs	Refactored iteration logic and updated HistoListUpdateHead to require an index parameter.
src/ImageSharp/Formats/Webp/Lossless/CostModel.cs	Replaced loop constructs with foreach for direct reference iteration.
src/ImageSharp/Formats/Webp/Lossless/BackwardReferenceEncoder.cs	Updated loops to use ref iteration and replaced clear methods for consistency.

Comments suppressed due to low confidence (2)

src/ImageSharp/Formats/Webp/Lossless/BackwardReferenceEncoder.cs:782

• [nitpick] Ensure that using ref iteration to mutate elements of a readonly struct is intended and safe in this context.

foreach (ref PixOrCopy v in refs)

src/ImageSharp/Formats/Webp/Lossless/HistogramEncoder.cs:467

• [nitpick] Verify that passing the index 'j' to HistoListUpdateHead correctly maintains the intended order of histogram pairs.

HistoListUpdateHead(histoPriorityList, p, j);

[Copilot]

Consider adding a boundary check to ensure 'this.count' does not exceed the allocated memory length to prevent potential overflows.

Suggested change

	public void Add(PixOrCopy pixOrCopy) => this.refs.Memory.Span[this.count++] = pixOrCopy;
	public void Add(PixOrCopy pixOrCopy)
	{
	if (this.count >= this.refs.Memory.Length)
	{
	throw new InvalidOperationException("Cannot add more elements. Memory limit exceeded.");
	}
	this.refs.Memory.Span[this.count++] = pixOrCopy;
	}