Update plugin elements with the new GstVaapiVideoMeta API.
This also fixes support for subpictures/overlay because GstVideoDecoder
generates a sub-buffer from the GstVaapiVideoBuffer. So, that sub-buffer
is marked as read-only. However, when comes in the textoverlay element
for example, it checks whether the input buffer is writable. Since that
buffer read-only, then a new GstBuffer is created. Since gst_buffer_copy()
does not preserve the parent field, the generated buffer in textoverlay
is not exploitable because we lost all VA specific information.
Now, with GstVaapiVideoMeta information attached to a standard GstBuffer,
all information are preserved through gst_buffer_copy() since the latter
does copy metadata (qdata in this case).
