... because it tests something that's experimental anyway, and the test is rubbish.

This leads to a big improvement in speed when (ab)using max_adu for gaps in the detector.

Since these will probably be around for a while...

cairo_surface_show_page() is only available from Cairo 1.6.0 onwards, but cairo_show_page() is just as easy
and is available from 1.2.0.