gst/base/gstbasetransform.*: Cache caps unit_size.

gst/base/gstbasetransform.*: Cache caps unit_size.

Original commit message from CVS:
* gst/base/gstbasetransform.c: (gst_base_transform_init),
(gst_base_transform_transform_size),
(gst_base_transform_configure_caps),
(gst_base_transform_get_unit_size),
(gst_base_transform_buffer_alloc),
(gst_base_transform_change_state):
* gst/base/gstbasetransform.h:
Cache caps unit_size.
Make sure we cannot negotiate up and downstream at the
same time.