vaapidecode: handle flush() vmethod

vaapidecode: handle flush() vmethod

Since GStreamer 1.2 the vmethod reset() in GstVideoDecoderClass was deprecated
and flush() was added.

This patch set the vmethod flush() if the installed GStreamer version is 1.2 or
superior. Otherwise, reset() is set.

v2: 1) In order to avoid symbol collision, the old method gst_vaapidecode_flush()
       was renamed to gst_vaapidecode_internal_flush().
    2) The new vmethod flush() always do a hard full reset.
v3: 1) Call gst_vaapidecode_internal_flush() first in flush() vmethod, in order to
       gather all collected data with  gst_video_decoder_have_frame()

https://bugzilla.gnome.org/show_bug.cgi?id=742922

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Signed-off-by: Sreerenj Balachandran <sreerenj.balachandran@intel.com>