rtp_engine: fix rtp payloads copy and improve argument names

rtp_engine: fix rtp payloads copy and improve argument names

In function ast_rtp_instance_early _bridge_make_compatible the
use of instance 0/1 as arguments doesn't clearly communicate a
direction that the copying of payloads from the source channel
to the destination channel will occur, making it more probable
to have the arguments to ast_rtp_codecs_payloads_copy() put in
the reverse order.  This patch renames the arguments with _dst
and _src suffixes and corrects the copy direction.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@402000 65c4cc65-6c06-0410-ace0-fbb531ad65f3