General cleanup in sequencer:

- Seperated StripData into
  StripData
  TStripData
  where StripData holds only image-filenames and TStripData holds
  the working information needed for ImBuf caching.
  => Large drop in memory usage, if you used a lot of movie and meta strips.
  => Fixed bugs in "duplicate" on the way (imbufs where copied around without
     taking reference counting seriously...)
  => Code is much cleaner now
- Added defines for TStripData->ok
  Finally figured out, what the magic values ment and named them properly :)
- Got rid of Sequence->curelem.
  Reason: very bad idea(tm) for multi threading with more than one render
  thread. Still not there, but this was a real show stopper on the way.

source/blender/include/BSE_sequence.h

@@ -37,6 +37,7 @@
 
 struct PluginSeq;
 struct StripElem;
+struct TStripElem;
 struct Strip;
 struct Sequence;
 struct ListBase;
@@ -44,9 +45,9 @@ struct Editing;
 struct ImBuf;
 struct Scene;
 
-void free_stripdata(int len, struct StripElem *se);
+void free_tstripdata(int len, struct TStripElem *se);
 void free_strip(struct Strip *strip);
-void new_stripdata(struct Sequence *seq);
+void new_tstripdata(struct Sequence *seq);
 void free_sequence(struct Sequence *seq);
 void build_seqar(struct ListBase *seqbase, struct Sequence  ***seqar, int *totseq);
 void free_editing(struct Editing *ed);
@@ -57,17 +58,20 @@ void clear_scene_in_allseqs(struct Scene *sce);
 
 int evaluate_seq_frame(int cfra);
 struct StripElem *give_stripelem(struct Sequence *seq, int cfra);
+struct TStripElem *give_tstripelem(struct Sequence *seq, int cfra);
 void set_meta_stripdata(struct Sequence *seqm);
 struct ImBuf *give_ibuf_seq(int rectx, int recty, int cfra, int chansel); 
 /* chansel: render this channel. Default=0 (renders end result)*/
+struct ImBuf *give_ibuf_seq_direct(int rectx, int recty, int cfra,
+				   struct Sequence * seq);
 
 /* sequence prefetch API */
 void seq_start_threads();
 void seq_stop_threads();
 void give_ibuf_prefetch_request(int rectx, int recty, int cfra, int chanshown);
 void seq_wait_for_prefetch_ready();
-struct ImBuf * give_ibuf_threaded(int rectx, int recty, int cfra, 
-				  int chanshown);
+struct ImBuf * give_ibuf_seq_threaded(int rectx, int recty, int cfra, 
+				      int chanshown);
 
 
 void free_imbuf_seq_except(int cfra);