libs/gst/base/gstbasetransform.c: Work around the brokenness of the event vmethod in basetransform. Prefer to return ...

Original commit message from CVS:
* libs/gst/base/gstbasetransform.c:
(gst_base_transform_sink_event):
Work around the brokenness of the event vmethod in basetransform. Prefer
to return TRUE when the subclass returned FALSE (meaning don't forward
the event).
* libs/gst/base/gstbasetransform.h:
Clarify the docs.