[Rd] Cairo bitmap output should work without X11 dependency (PR#12218)

[cruff at ucar.edu]

Full_Name: Craig Ruff
Version: 2.7.1
OS: Irix 6.5.30
Submission from: (NULL) (128.117.8.145)


Cairo based bitmap graphics output should work if cairo and R have been compiled
without xlib support.  2.7.1 gives the "X11 is not available" message in this
case when run interactively, and from my reading the png documentation, calling
png(filename=..., type="cairo") should work.  Cairo itself is happy without
xlib.