Fix degeneracy check for resize transforms

[JimBobSquarePants]

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Description

Fixes #3156

Use exact zero determinant checks in TransformUtilities.IsDegenerate for Matrix3x2 and Matrix4x4 instead of epsilon-based near-zero detection, preventing valid transforms from being treated as degenerate during extreme downscales. Added a regression test for issue #3156 to verify resizing large square images (1024/2048) to 1x1 succeeds and preserves the expected pixel.

[Copilot]

Pull request overview

Fixes a regression where very small-but-valid resize transforms (eg 2048×2048 → 1×1) were incorrectly treated as degenerate, causing DegenerateTransformException during resize operations.

Changes:

• Updated TransformUtilities.IsDegenerate for Matrix3x2 and Matrix4x4 to treat matrices as degenerate only when the determinant is exactly zero (instead of near-zero via epsilon).
• Removed the now-unused epsilon-based IsZero(float) helper from TransformUtilities.
• Added a regression test covering resizing large square images (1024/2048) down to a single pixel and validating the resulting pixel value.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeTests.cs	Adds a regression test ensuring large-square → 1×1 resize succeeds and preserves expected output.
src/ImageSharp/Processing/Processors/Transforms/TransformUtilities.cs	Adjusts degeneracy detection to avoid false positives for extreme downscales.

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