gst/gstcaps.c: Add new function.

gst/gstcaps.c: Add new function.

Original commit message from CVS:
* gst/gstcaps.c: (_gst_structure_is_equal_foreach),
(gst_caps_is_equal_fixed): Add new function.
* gst/gstcaps.h: ditto.
* gst/gstpad.c: (gst_real_pad_class_init),
(gst_pad_link_call_link_functions), (gst_pad_try_set_caps),
(gst_pad_set_explicit_caps), (gst_pad_get_caps):  In try_set_caps,
check new caps against existing caps -- if they're the same, return
OK without renegotiating.  caps-nego-failed signal fixed so that
the marshaller isn't VOID__OBJECT.  Also changed to G_TYPE_POINTER
to save an extra caps copy.  Don't complete negotiation if a pad
link function returns DELAYED.