cogl_rectangle has some validation code to check whether the first
layer has a sliced texture. If so it will abandon the rest of the
layers and print a warning. However it was even doing this pruning and
displaying the warning if there is only one layer. This patch just
makes it check whether the pipeline actually has more than one layer
before pruning or displaying the warning but it will still fallback to
the multiple quads path.

Reviewed-by: Robert Bragg <robert@linux.intel.com>