primitive: Don't leak indices
primitive: Don't leak indices

If a CoglPrimitive is associated with a set of indices then we must
unref those indices when freeing the primitive to avoid a leak.

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

djdeath/cogl,djdeath/cogl-android,djdeath/cogl-android,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl,gcampax/cogl,djdeath/cogl,gcampax/cogl