Fix axis aligned gaussian causing NaN in shader

[LioQing]

Finally fixed this annoying issue, it is caused by axis aligned Gaussian position (which is why the coverage/model.ply did not have this problem, because the Gaussian positions are not axis aligned), causing the eigenvector to be 0, 0, which gave an NaN when normalizing

Closes #8

[Copilot]

Pull request overview

This PR fixes a critical rendering bug where axis-aligned Gaussian positions caused NaN values in the shader during eigenvector normalization. The issue occurred when the 2D covariance matrix had zero off-diagonal terms and equal eigenvalues, resulting in a zero-length eigenvector that produced NaN when normalized.

Changes:

• Added zero-vector check before normalization in cov2d_axes function to prevent NaN values
• Updated coverage data (auto-generated)

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/shader/utils.wesl	Fixed NaN bug by using select to provide fallback direction when eigenvector is zero
coverage/lcov.info	Auto-generated code coverage data update

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