Highlight outline depth test issue: transparent meshes and occlusion

## Description

The highlight/outline effect has depth testing issues that cause incorrect rendering in two scenarios:

1. **Transparent mesh highlighting**: When a transparent mesh is highlighted, the outline appears on objects behind it instead of on the transparent mesh itself
2. **Occluded object highlighting**: When highlighting an object behind another object, the front (occluding) object appears to be highlighted instead

## Root Cause

The outline shader is likely using `depth != neighbor_depth` to detect edges, which triggers on depth discontinuities in both directions (front-to-back and back-to-front).

## Proposed Fix

Change the depth comparison to `depth > neighbor_depth` instead of `depth != neighbor_depth`. This will only draw outlines where the current pixel is farther than its neighbor, avoiding false positives from objects behind the highlighted mesh.

## Related

- Reported with screenshot showing transparent mesh outline issue

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Requested by rob via Slack