DeckLink sync mode: preroll 4 frames

3 frames doesn't seem to be sufficient with DeckLink HD Extreme 3D+
when using compression (interestingly without it it seem to work with 3).

src/video_display/decklink.cpp

@@ -88,7 +88,7 @@
 
 enum {
         SCHED_RANGE              = 2,
-        DEFAULT_MIN_SCHED_FRAMES = 3,
+        DEFAULT_MIN_SCHED_FRAMES = 4,
         DEFAULT_MAX_SCHED_FRAMES = DEFAULT_MIN_SCHED_FRAMES + SCHED_RANGE,
         MAX_UNPROC_QUEUE_SIZE    = 10,
 };