Usually pango_layout_get_pixel_extents() is used, which uses
PANGO_PIXELS_CEIL on the rectangle's width.  This commit makes the new
function gtk_cell_renderer_text_get_preferred_width() consistent with
this.

This fixes rounding errors on Mac OS X, where we were seeing tree views
with a double height for a single line of text, while the usual single
row height would have been sufficient.