CAIRO_LINE_CAP_ROUND is the same as in FIG mode.
Note that all non-dashed lines disappear when generating PDF and
CAIRO_LINE_CAP_ROUND is set. For now, we just use CAIRO_LINE_CAP_SQUARE.
With 2, lines were far too thick. Note that the 0.5 points (1/72 in) we
have now are roughly twice what XFig uses (1/160 in for PDF or PNG),
yet things don't look very fat there.