Bad extents for some glyphs

[matthiasclasen]

I've been playing with making single-glyph icon fonts.

Here is one: (gzipped to make github accept it)
adwaita.ttf.gz

The problem is that I get bad extents for some of the glyphs (the actual icon is fine, but the components are not).

The working glyph is 2, a bad one is 3:

$ hb-shape --show-extents ~/adwaita.ttf --glyphs [gid2=0+1000]
[icon0=0+1000<180,960,1060,-1220>]

$  hb-shape --show-extents ~/adwaita.ttf --glyphs [gid3=0+1000]
[icon0.0=0+1000<0,-1,-1,1>]

This may be because glyph2 has a ClipBox, and the others don't.