A small bug in yesterday's work: since in Cairo mode
draw_stretch_before changes the transformation matrix, if we do it
before calling draw_clip then the clip region will be interpreted in
the transformed coordinates.
This caused a subtle display bug in yesterday's commit: drawing one
half of double-height text would have drawn _both_ halves of it on to
the window's backing pixmap, but only copied the correct half on to
the window proper - but the overdrawing on the pixmap would have shown
up if the window was hidden and re-exposed.
828ad5d6d4