Configure DVB-Subtitle Appsink to Sync mode.
Configure DVB-Subtitle Appsink to Sync mode.

:Release Notes
DVB-subtitle app sink configured as Sync mode based on the
caps information. Similar to DVB Teletext Subtitle.

:Issue Addressed
1. cellcom_dvbstitle.ts - subtitle text is shown and disappears fast
2. Alwasy subtitle first frame failed display
3. Play  and change the DVB-subtitle track, Track was not changed.
4. Pause and change the DVB-subtitle track, Track was not changed.
5. After FWD/RWD seek some subtitle frames were missed.
6. Repeated the DVB subtitle testing with same stream 6 or 7 th time
DVB subtitle was not shown.

:Test Performed
Tested on H15 target and LM15U Target

Change-Id: Id6df5c0d119463b0219728917fc3991fff5854d9
Reviewed-on: https://gpro.lgsvl.com/77509
Reviewed-by: HoonHee Lee <41497318ef67053f37c2ac0e974a3369f66d66ec@lge.com>
Reviewed-by: Wonchul Lee <9841bdbec9dbb2acc2dcaaa1dab0491399f0e464@lge.com>
Tested-by: Wonchul Lee <9841bdbec9dbb2acc2dcaaa1dab0491399f0e464@lge.com>

justinjoy/gst-plugins-cool,justinjoy/gst-plugins-cool