cairo: Only apply options to table items, to avoid crash.

Most of the members of struct xr_rendering, including the 'xr'
member, are used only for table items.  In chart items, these
members are zeroed.  But xr_rendering_apply_options() attempted to
use the 'xr' member even for chart items, which caused a crash.

This reverts and replaces the provisional fix already applied by
John Darrington in commit ee7b576822f (Avoid crash rendering charts
in the gui interface.), which also fixed the crash but added extra
unnecessary work.

Reported-by: Nick Macdonald <nick@nickmacdonald.net>