Globe occlusion for popups and improving marker performance on globe

[SnailBones]

Launch Checklist

#11556 added support for marker and popup position on globe and marker occlusion behind globe. This PR adds occlusion for markers that are not attached to a popup.

Also improves the occlusion algorithm as described here: #11556 (review)

Also fixes a minor bug where a marker behind terrain and in fog would have its fog opacity calculated at the point of the intersecting terrain, not the marker itself. i.e. in some cases marker opacity would increase when the marker was hidden behind terrain.

Other changes are refactoring for code readability and removing repetition.

A future optimization could include caching the result to reduce calls to latLngToECEF, though this may not be worth the added complexity.

• Globe occlusion for markeless popups.
• Fog support + terrain occlusion for markerless popups.
• 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 markers in fog sometimes becoming more visible when behind terrain. </changelog>

[karimnaaji]

Looks good @SnailBones ! I've left a few nits, but the one missing thing before merging is adding a small test for popup occlusion, otherwise good to merge at your convenience.