cairo: Fix failures to scale between cairo and pango units.

The difference between Cairo and Pango units, as currently defined by
XR_POINT and PANGO_SCALE, is only 2.4% (1000 versus 1024).  This makes the
difference rarely noticeable, and easy to miss when casually looking at
output.  However, while continuing to develop the PSPP output drivers, I
did notice.

To make the difference obvious, I temporarily changed XR_POINT from 1000
to 500.  This commit fixes the problems that this revealed.