rtpsession: don't unref buffer twice

Cleaning the packet info will already unref the buffer.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=715078
videomixer: explicitly fail when alpha information would have been lost.
videomixer: Fix segfault when filling the background of a UYVY frame

https://bugzilla.gnome.org/show_bug.cgi?id=712401
pulsesink: Free device_info in accepts caps

https://bugzilla.gnome.org/show_bug.cgi?id=719811
check: fix jitterbuffer check

Don't advance the clock to 240ms too early.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=710013