avformat/rtsp: move SDP_MAX_SIZE macro definition to header file
move comments for the size of SDP_MAX_SIZE here:
Some SDP lines, particularly for Realmedia or ASF RTSP streams,
contain long SDP lines containing complete ASF Headers (several
kB) or arrays of MDPR (RM stream descriptor) headers plus
"rulebooks" describing their properties. Therefore, the SDP line
buffer is large.
The Vorbis FMTP line can be up to 16KB - see xiph_parse_sdp_line
in rtpdec_xiph.c.
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>