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)
14 jobs for rtpvrawpay-set-discont-flag-on-first-buffer in 59 minutes and 58 seconds (queued for 39 seconds)
Status Job ID Name Coverage
  Preparation
passed #224396
gst indent

00:00:30

passed #224395
manifest

00:00:37

 
  Build
passed #224397
build fedora x86_64

00:10:11

passed #224398
build nodebug fedora x86_64

00:06:54

 
  Test
passed #224399
check fedora

00:04:54

passed #224400
integration testsuites fedora

00:15:01

 
  Full Builds
passed #224402
build cerbero android universal

00:12:00

passed #224403
build cerbero cross win32

00:09:01

passed #224404
build cerbero cross win64

00:04:22

passed #224401
build cerbero fedora x86_64

00:05:31

 
  Full Tests
passed #224406
valgrind bad

00:05:29

passed #224596
valgrind good

00:08:36

failed #224405
valgrind good

00:15:01

 
  Apps
passed #224407
android universal examples

00:07:45