Fix missing tiles at bottom of screen during globe-Mercator transition

[SnailBones]

Closes https://github.com/mapbox/gl-js-team/issues/443

All changes to functionality are in 34762b0, previous commits are refactoring from #11987

Ported from https://github.com/mapbox/mapbox-gl-native-internal/pull/3450

Fix 1

• Description: Rendering artefacts (missing tiles) when the zoom level is just below the threshold for mercator projection. Most prominent with high pitch views where tiles at the bottom of the screen might disappear.
• Cause: Visible tiles on the screen are found by comparing whether the camera frustum is intersecting with axis aligned bounding boxes (aabb) of tiles. Prior to this PR aabbs of tiles were not interpolated during the transition between globe and mercator: either mercator aabbs or globe aabb were used, nothing in between. This is why these issues were appearing on zoom levels just below 6.
• Fix: Compute aabbs from interpolated tile vertices.

The issue in GL JS, in this case with a pole circle visible:

Test before fix on left, after fix on right:

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
• port wrapping
• 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 occasional missing tiles at bottom of screen during globe-Mercator transition</changelog>

[karimnaaji]

LGTM after addressing review comments from #11987 and a rebase.

[karimnaaji]

👍