vaapipostproc: reset deinterlacer state when there is a discontinuity.

vaapipostproc: reset deinterlacer state when there is a discontinuity.

Reset deinterlacer state, i.e. past reference frames used for advanced
deinterlacing, when there is some discontinuity detected in the course
of processing source buffers.

This fixes support for advanced deinterlacing when a seek occurred.

https://bugzilla.gnome.org/show_bug.cgi?id=720375

[fixed type of pts_diff variable, fetch previous buffer PTS from the
 history buffer, reduce heuristic for detecting discontinuity]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>