Rendering issue with triangle vertices clipped in viewport with Phong lighting

[Bigfoot71]

There appears to be a problem with the vertices of triangles clipped within the viewport when using Phong lighting. This issue persists both in barycentric rendering mode and scanline mode.

The problem does not seem to originate from normal interpolation during clipping. In the raylib_FirstPerson example, normals consistently point towards either 1 or -1 along a single axis. Even when printing them to the console or displaying them on screen with color, everything appears normal.

I also do not see any issues with position interpolation.

This problem mostly manifests when triangles are clipped according to the viewport.

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Screenshots of the issue:

Image rendering normals with color at locations where triangles are clipped, showing no apparent issues: