avformat/rtmpproto: only prepend @setMetaData for onMetaData and |RtmpSampleAccess

In current versions of ffmpeg, when streaming to an RTMP server, anytime a packet of type
RTMP_PT_NOTIFY is encountered, the packet is prepended with @setDataFrame before it gets sent
to the server. This is incorrect; only packets for onMetaData and |RtmpSampleAccess should
invoke @setDataFrame on the RTMP server. Specifically, the current bug manifests
itself when trying to stream onTextData or onCuePoint invocations.

This fix addresses that problem and ensures that the @setDataFrame is only prepended
for onMetaData and |RtmpSampleAccess.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>