gst/base/gstbasetransform.*: Make passthrough work using the bufferpools.

gst/base/gstbasetransform.*: Make passthrough work using the bufferpools.

Original commit message from CVS:
* gst/base/gstbasetransform.c: (gst_base_transform_init),
(gst_base_transform_transform_caps), (gst_base_transform_getcaps),
(gst_base_transform_configure_caps), (gst_base_transform_setcaps),
(gst_base_transform_get_size), (gst_base_transform_buffer_alloc),
(gst_base_transform_handle_buffer), (gst_base_transform_getrange),
(gst_base_transform_chain), (gst_base_transform_change_state),
(gst_base_transform_set_passthrough),
(gst_base_transform_is_passthrough):
* gst/base/gstbasetransform.h:
Make passthrough work using the bufferpools.
Changed API a bit, subclasses have to write into a buffer
provided by the base class.
More debug info in nego functions.

* gst/elements/gstidentity.c: (gst_identity_init),
(gst_identity_transform):
Port to new base class.