filter_chain: rework filter_chain_VideoDrain()

From the last filter in the chain to the first we:

- drain to get a picture
- if there is a picture we use it to filter it
- if we have a picture from running all the filters, we return it
- otherwise we try again to drain the filter that produced a picture
- if that filter doesn't produce a picture anymore, we try the previous filter
  in the chain

src/misc/filter_chain.c

@@ -468,16 +468,26 @@ picture_t *filter_chain_VideoFilter( filter_chain_t *p_chain, picture_t *p_pic )
 
 picture_t *filter_chain_VideoDrain( filter_chain_t *p_chain )
 {
-    picture_t *p_pic;
-    for( chained_filter_t *b = p_chain->last; b != NULL; b = b->prev )
+    for( chained_filter_t *b = p_chain->last; b != NULL; )
     {
-        if( vlc_picture_chain_IsEmpty( &b->pending ) )
+        picture_t *p_pic = filter_DrainVideo( &b->filter );
+        if ( p_pic )
+        {
+            // we have a picture, feed it down the chain
+            p_pic = filter_chained( b->next, p_pic );
+
+            if ( p_pic )
+                // all filters produced a picture, we can return it
+                return p_pic;
+
+            // b drains picture, keep draining pictures until it's dry
+            // we assume the next filters won't produce a picture since
+            // they didn't produce one with a regular picture
             continue;
-        p_pic = vlc_picture_chain_PopFront( &b->pending );
+        }
 
-        p_pic = FilterChainVideoFilter( b->next, p_pic );
-        if( p_pic )
-            return p_pic;
+        // we don't have a picture from b, drain from further up in the chain
+        b = b->prev;
     }
     return NULL;
 }