rtspsrc: Fix deadlock when task creation is no successful

https://bugzilla.gnome.org/show_bug.cgi?id=725124
jpegdec: mark all parsed frames as sync points

all jpeg frames are sync points, so mark them as such so
reverse playback can properly work with the video decoder
base class

https://bugzilla.gnome.org/show_bug.cgi?id=725104
qtdemux: prevent segfault after seeking with negative rate

After a seek, the position and segments info of qtdemux are
reset to null or invalid values. For reverse playback, qtdemux
will seek to a keyframe and play until the next keyframe, then
seek to another previous keyframe. This seeking function requires
the position and segments info of qtdemux to be properly initialized.

Due to a missing check for segment.rate in the movie demuxing loop
it would consider the current segment to have ended without having
it initialized, leading qtdemux to attempt to do a previous keyframe
seek without proper position and segment info data. This would cause
 a segfault. This patch fixes it by correctly checking for segment
boundaries taking the segment.rate into consideration.

https://bugzilla.gnome.org/show_bug.cgi?id=725104