rtpvrawpay: preserve GST_BUFFER_FLAG_DISCONT on the first outputted buffer

If the incoming frame buffer has GST_BUFFER_FLAG_DISCONT set this should
be preserved and set for the first output buffer too, like other
payloaders do.

Spotted with gst-validate-1.0 when adding integration tests for
rtpsession, a minimal test to reproduce the issue is:

$ gst-validate-1.0 videotestsrc num-buffers=1 ! rtpvrawpay ! identity ! fakesink
Starting pipeline
Pipeline started
   warning : Buffer didn't have expected DISCONT flag333 speed: 1.000000 />
             Detected on <identity0:sink>
             Detected on <identity0:src>
             Detected on <fakesink0:sink>
             Description : Buffers after SEGMENT and FLUSH must have a DISCONT flag

Issues found: 1

=======> Test PASSED (Return value: 0)

0 files changed, 0 insertions, 0 deletions