plugins/elements/gstidentity.*: Separate out check-imperfect-timestamp and check...

plugins/elements/gstidentity.*: Separate out check-imperfect-timestamp and check-imperfect-offset.

Original commit message from CVS:
* plugins/elements/gstidentity.c: (gst_identity_class_init),
(gst_identity_init), (gst_identity_check_perfect),
(gst_identity_check_imperfect_timestamp),
(gst_identity_check_imperfect_offset), (gst_identity_transform_ip),
(gst_identity_set_property), (gst_identity_get_property):
* plugins/elements/gstidentity.h:
Separate out check-imperfect-timestamp and check-imperfect-offset.
Put back check-perfect as it was to keep compatibility.