Add copy_metadata function to imxipuvideotransform

This ensures no input metadata is copied over, only the timestamps and
flags. Furthermore, the GST_BUFFER_FLAG_TAG_MEMORY flag isn't copied.
This is fixed in gstreamer git upstream
(rev f0deb37a8697b7a01d2f935949ce3e1412c1293c), but to make gstreamer-imx
work with 1.4.1 and older, this check is included.

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>