Reworked the way the vout select the pictures to be displayed.

It allows to:
 - properly support filters that do not have 1:1 input/output relation
 (yadif2x, bob, ... are back).
 - filter only once each picture (for the 'static' filter chain). It
 fixes filters with state behavior.
 - display as soon as possible the first frame after a flush.

There are still a few issues:
 - when paused, on filter changes, the picture displayed step forward the
 first time.
 - preventive frame dropping is not enough agressive.

src/video_output/vout_internal.h

@@ -48,12 +48,6 @@
  */
 #define VOUT_MAX_PICTURES (20)
 
-/**
- * Number of frames used to estimate the maximum filter chain latency.
- * For performance, it is best to use a power of 2
- */
-#define VOUT_FILTER_DELAYS (8)
-
 /* */
 struct vout_thread_sys_t
 {
@@ -107,6 +101,8 @@ struct vout_thread_sys_t
         int         qtype;
         bool        is_interlaced;
         picture_t   *decoded;
+        picture_t   *current;
+        picture_t   *next;
     } displayed;
 
     struct {
@@ -134,8 +130,6 @@ struct vout_thread_sys_t
         vlc_mutex_t     lock;
         filter_chain_t  *chain_static;
         filter_chain_t  *chain_interactive;
-        unsigned        delay_index;
-        mtime_t         delay[VOUT_FILTER_DELAYS];
     } filter;
 
     /* */