spans: avoid normalize with normalize_factor of 1
spans: avoid normalize with normalize_factor of 1

if a normalize factor of 1 is passed then we don't need to normalize the
starting point to find the closest point equivalent to 0.

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

djdeath/cogl-android,collects/cogl,Distrotech/cogl,Distrotech/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl,djdeath/cogl-android,djdeath/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl,gcampax/cogl,collects/cogl