buffer: add gst_buffer_new_copy() convenience function

More convenient and discoverable variant of the fairly widely-used
gst_buffer_new_wrapped(g_memdup(data,size),size).

Part-of: <gstreamer/gstreamer!826>
39 jobs for !826 with gst-buffer-new-copy in 27 minutes and 4 seconds (queued for 3 seconds)
merge request