Render test: Inconsistent fill extrusion height rendering across tile boundaries

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**mapbox-gl-js version**: v2.0.0

**browser**: Chrome but likely any

### Steps to Trigger Behavior

This was found during work on running the render tests in the browser. The affected tests are `fill-extrusion-pattern/multiple-layers-flat` and `fill-extrusion-pattern/opacity-terrain-flat-on-border`.

1. Remove `"render-tests/fill-extrusion-pattern/opacity-terrain-flat-on-border": "skip"` from `test/ignores.json`
2. Run `yarn watch-render`
3. Visit `http://localhost:7357/849/test/integration/testem_page.html?filter=fill-extrusion-pattern/opacity-terrain-flat-on-border`

### Link to Demonstration

n/a

### Expected Behavior

The tests should pass on every run.

### Actual Behavior

The tests occasionally fail because the building heights are rendered higher than expected. This only seems to happen with specific buildings at tile boundaries. I tried inlining the terrain rather than calling a `setTerrain` operation but that didn't fix the issue.

This is related to https://github.com/mapbox/mapbox-gl-js/issues/10170 but I believe exhibits somewhat different behavior (building height vs flat roofs).

Start with the render tests named in the issue, especially fill-extrusion-pattern/opacity-terrain-flat-on-border, and inspect its skip entry in test/ignores.json. Run yarn watch-render and the provided testem URL to reproduce the intermittent tile-boundary rendering difference. Done means the affected render tests pass consistently without the skip.

Written by the indexing model from the issue text.