Glyph node: Those null coords are useful after all

Artifacts were visible on Linux after 3f88a5b835198e39
and this seems to be the only behavior change hitting
non-retina displays.

Change-Id: I0d4dc5ca05adb4585c6afad2a24d9c3a8551d809
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>