fix: pan the globe from the center when the grab is off the sphere

[mondsichtung]

Dragging the globe from the empty space around it barely moves the map and often in the wrong direction; releasing then flings it fast the way the cursor actually moved, because the inertia only looks at the pixel offset.

JSBin

This regressed in #8154: panning makes the grabbed surface point follow the cursor. To reproduce on its demo, https://jcolot.github.io/maplibre-gl-js/, grab the empty space top right of the globe with the mouse and drag left. Off the sphere there is no such point, so a falloff substitutes one near the globe's edge, and dragging mostly swings that point around the center, a motion panning with a fixed bearing discards; only the small part of the drag aimed at or away from the globe still pans.

Grabs that miss the sphere now pan from the screen center, the same fallback the gesture's zoom already uses in HandlerManager._applyChanges. Off the globe the map now moves a pixel per pixel in the drag direction, matching the inertia; grabs on the globe are unchanged (#5296).

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
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Assisted-By: Claude Code (Opus 5, xhigh)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (ef90e66) to head (defdd01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8174      +/-   ##
==========================================
- Coverage   93.92%   93.91%   -0.01%     
==========================================
  Files         290      290              
  Lines       24916    24917       +1     
  Branches     6575     6576       +1     
==========================================
- Hits        23402    23401       -1     
- Misses       1514     1516       +2     

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[HarelM]

Can you share a before and after video maybe? Or an interactive site that has the old and new behavior side by side?
CC: @jcolot

[mondsichtung]

Attached both a GIF as well as a JSBin URL.