Sets video mark bit on format field correctly

This fixes a regression in the media architecture change
where video frames did not have their video mark set
correctly.  dvossel wrote this. twilson kindly committed
this, mmichelson found the bug.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@313822 65c4cc65-6c06-0410-ace0-fbb531ad65f3