baseparse: do not accumulate buffers on passthrough mode
baseparse: do not accumulate buffers on passthrough mode

If on passthrough during reverse playback, do not accumulate buffers as
baseparse will never check for DISCONT flag to push those buffers.
So just push buffers downstream as if it was forward playback.

https://bugzilla.gnome.org/show_bug.cgi?id=721941

justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer