atlas: make zero size a programmer error
atlas: make zero size a programmer error

Instead of throwing a CoglError exception if an application tries to
allocate a zero size atlas texture this make that a programmer error
instead.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

djdeath/cogl,djdeath/cogl,djdeath/cogl