Split RMContext into RMDemux/MuxContext and make them private in rmdec/enc.c.

Reason for this is that there are no shared entries in the demuxer/muxer
context, making it a mystery as to why it was shared between the two. See
"[PATCH] clean rmdemux/muxcontext" patch on mailinglist.

Originally committed as revision 16111 to svn://svn.ffmpeg.org/ffmpeg/trunk