Fix a potential incorrect vertex buffer access

[karimnaaji]

A user reported a rendering artifact with a custom data set of fill-extrusions. The proposed PR prevents the centroid vertex buffer to be filled for some features but not others (when some features fall through the flat roof check but not others), eventually leading to mismatching sizes between the fill-extrusion vertex buffer and its associated centroid buffer. This prevents access to unassigned gpu data that leads to the rendering artifact this user is experiencing.

121939782-394e6600-cd02-11eb-86a2-e15048197791.mp4

The regression render test manifests the assertion on main the branch, and goes through with this fix:

>  mapbox-gl-js git:(main) ✗ yarn run test-render tests=regressions/mapbox-gl-js-team#217

$ SUITE_NAME=render CI=true testem ci -f test/integration/testem/testem.js 'tests=regressions/mapbox-gl-js-team#217'
    ---
        browser log: |
            testContext: [object Object]
            ERROR: Uncaught AssertionError: false == true at blob:http://localhost:7357/30198dab-7e9d-421c-ba40-745ff656fa55, line 952
    ...
not ok 2 Chrome 91.0 - [undefined ms] - render-tests/regressions/mapbox-gl-js-team#217 timed out after 5000ms
    ---

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix a potential rendering artifact when using custom fill-extrusion data set with terrain</changelog>

[rreusser]

Looks good to me. I'm a little fuzzy on the precise details, but feels like the optimization could be kept in the case that it's all either one or the other, and that the issue is the mismatch arising from a little bit in both branches. Unless there's a clear performance win by making the fix fancier, this seems 💯 .

[arindam1993]

👍🏽