area outside `content` doesn't work with collision detection

[ansis]

mapbox-gl-js version: master

browser: all

Steps to Trigger Behavior

1. open debug/stretchable.html
2. zoom out

Link to Demonstration

debug/stretchable.html

Expected Behavior

Symbols disappear as soon as any part of the icon overlaps.

Actual Behavior

Symbols only disappear when the text or icon's content overlap.

[ansis]

fixed by #9041