- Fix segmentation fault in prebuffering
Md: could you please review this?

This breaks when the sender uses a fancy payload type, or RTCP-mux.

- Fix segfault if there is no preparse and/or fetcher object

One should never signal a condition without the corresponding lock:
 "if predictable scheduling behavior is required, then that mutex shall
  be locked    by    the    thread   calling   pthread_cond_broadcast()
  or pthread_cond_signal()." -- POSIX

This only fixes the most obvious cases. There might be others.

Get correct major opcode for MIT-SHM instead of an hard-coded one that
works with some X servers (not including mine).

mode by default. Packagers and build bots should re-enable it with:
./configure --enable-fast-install

queuing any data - this is mostly useful when quitting

 - vlc_object_lock, vlc_object_unlock: take/release object_lock
 - vlc_object_signal: trigger object_wait
 - vlc_object_wait, vlc_object_timedwait: wait on object_wait