theoraenc: add "dup-on-gap" option

theoraenc: add "dup-on-gap" option

This option will produce duplicate frames if we get
a frame with GAP flag. This will reduce CPU load and file size.

This option should be disabled for real time applications, because it
collects GAP frames and waits until it gets a non GAP frame to start
encoding.

v30.06.2011: make some spell changes.
v03.07.2011: add handling of EOS and discontinuous for dup-on-gap.
v19.12.2011: fix pointer dangling in theora_timefifo_free
v20.12.2010: fix timestamp bug for dup-on-gap=0

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=627459

Signed-off-by: Oleksij Rempel (Alexey Fisher) <bug-track@fisher-privat.net>