Fix incorrectly shaded concave corner of fill extrusions when AO is enabled

[akoylasar]

When rounded edges and AO for fill extrusions are enabled simultaneously the concave corners between the walls are shaded incorrectly as seen below:

this PR resolves the issue and adds a render test verifying the fix:

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
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• 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 incorrect shading of corners in fill extrusions when ambient occlusion is enabled.</changelog>

[akoylasar]

@mapbox/gl-native @astojilj @ivovandongen decided to merge this to include a render test before proceeding with the native PR.