libs/gst/base/gstbasetransform.*: Add support for dropping buffers with custom GstFlo...

libs/gst/base/gstbasetransform.*: Add support for dropping buffers with custom GstFlowReturn.

Original commit message from CVS:
* libs/gst/base/gstbasetransform.c:
(gst_base_transform_sink_eventfunc),
(gst_base_transform_handle_buffer), (gst_base_transform_chain),
(gst_base_transform_activate):
* libs/gst/base/gstbasetransform.h:
Add support for dropping buffers with custom GstFlowReturn.
Set DISCONT flags on outgoing buffers based on QoS, incomming DISCONT
buffers or dropped buffers.
* docs/libs/gstreamer-libs-sections.txt:
docs for new custom return code.
* plugins/elements/gstidentity.c: (gst_identity_transform_ip):
Use drop support in base class to implement drop-probability.