Add setOffset to popup

[jutaz]

Launch Checklist

Adds Popup.prototype.setOffset which allows offset to be applied after popup is already created.

• 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
• document any changes to public APIs
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• manually test the debug page
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• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
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[mourner]

Thanks!