Fix line rendering when one endpoint is behind the camera.

[mrdoob]

Made with Claude Opus 4.5.

Problem

Lines with one endpoint behind the camera (w <= 0) were being incorrectly rendered or disappearing entirely. The perspective division (P.x / P.w) produces invalid values when w <= 0, causing the line's rectangle polygon to have garbage coordinates before clipping could occur.

Solution

Clip line endpoints to w > epsilon before computing the screen-space direction (dx, dy). This ensures valid perspective division and proper rectangle construction, allowing the subsequent polygon clipping to work correctly.

Test

The fix can be verified with three.js examples that render grid lines extending toward the camera (e.g., webgl_materials_car, webxr_vr_teleport). Before the fix, grid lines close to the camera would break/disappear. After the fix, they render continuously to the screen edges.

Before	After

[skal65535]

Looks like the dEQP test suite could use an extra test for this case of line rendering...

(https://github.com/KhronosGroup/VK-GL-CTS)