Don't create a pixmap for CT_ROOT and CT_OUTPUT containers.

Don't create a pixmap for CT_ROOT and CT_OUTPUT containers.

Such containers never require a pixmap anyway. In particular for the __i3
output (used for the scratchpad workspace), this would cause an allocation
error anyway because it can be very big -- so big, in fact, that X cannot
allocate the pixmap for it.

Until now, this error was silently ignored due to the fact that we did not
create the pixmap checked (and asserted its success), but with cairo this
would cause a crash because we'd try to create a surface for a pixmap
which doesn't exist.

relates to #1278