[FIX] Fix glTF node matrix decomposition with negative scale

[willeastcott]

Fixes #8328

Description

When a glTF node uses a matrix property (instead of separate translation, rotation, scale properties) that contains a negative scale component (creating a reflection/flip), the model was displayed incorrectly - appearing flipped or upside down.

Changes

• Use Quat.setFromMat4() for rotation extraction instead of Mat4.getEulerAngles() when decomposing node matrices
• Apply scaleSign to the X scale component to preserve negative determinant (mirrored) transforms

Technical Details

The root cause was that Mat4.getScale() returns only positive values (it computes vector lengths), and Mat4.getEulerAngles() doesn't coordinate with negative determinant handling. This could cause the flip to be absorbed into the rotation as an extra 180° rotation, or be lost entirely.

The fix uses Quat.setFromMat4() which already properly handles negative determinant matrices by normalizing the X axis before rotation extraction (see lines 589-597 in quat.js). The negative scale is then preserved by multiplying the X scale by Mat4.scaleSign, which returns -1 for mirrored matrices.

This approach is consistent with how the engine handles mirrored transforms elsewhere (e.g., batch manager, GraphNode._worldScaleSign).

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

This PR fixes incorrect rendering of glTF models when nodes use a matrix property containing negative scale components (reflections/flips). The issue was that the previous matrix decomposition approach using Mat4.getEulerAngles() didn't properly handle negative determinant matrices, causing models to appear flipped or inverted.

Key changes:

• Switched from Euler angle extraction to quaternion-based rotation extraction for better handling of mirrored transforms
• Applied scale sign correction to preserve negative determinant (mirrored) matrices

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[willeastcott]

@mvaligursky I know this already has 1 approval but I think it's best if I get your approval too since you commented on the original issue. Let me know if you think this fix is correct.