* include/stream_output.h, src/stream_output/stream_output.c: added a SOUT_MUX_CAP_GET_ADD_STREAM_WAIT muxer capability to specify that a muxer prefers to wait all ES before starting muxing.
* modules/mux/ogg.c: use SOUT_MUX_CAP_GET_ADD_STREAM_WAIT.
* modules/stream_out/transcode.c: fix for video stream header generation (theora).
* modules/codec/theora.c: don't overwrite headers.