Fixes for MovieClip strip user counter.

Currently copies behavior of clip and image editors:
- On file load, all strip will reference clip they're using
- On adding new strip, clip would be referenced only if it've got zero user
- On removing strip clip wouldn't be de-referenced to prevent clip editors
  pointing to zero-counted datablocks.
Not actually ideal from human beings point of view, but referencing/dereferencing
clip on each strip add/delete is getting crappy because of current logic of how
clip datablocks are referenced from clip editor (which is designed to work fine with
loading files without loading UI).

source/blender/blenloader/intern/readfile.c

@@ -4841,6 +4841,7 @@ static void lib_link_scene(FileData *fd, Main *main)
 				}
 				if (seq->clip) {
 					seq->clip = newlibadr(fd, sce->id.lib, seq->clip);
+					seq->clip->id.us++;
 				}
 				if (seq->scene_camera) seq->scene_camera= newlibadr(fd, sce->id.lib, seq->scene_camera);
 				if (seq->sound) {

source/blender/editors/space_sequencer/sequencer_add.c

@@ -50,6 +50,7 @@
 
 #include "BKE_context.h"
 #include "BKE_global.h"
+#include "BKE_library.h"
 #include "BKE_main.h"
 #include "BKE_sequencer.h"
 #include "BKE_movieclip.h"
@@ -323,7 +324,10 @@ static int sequencer_add_movieclip_strip_exec(bContext *C, wmOperator *op)
 	seq->type = SEQ_MOVIECLIP;
 	seq->blend_mode = SEQ_CROSS;
 	seq->clip = clip;
-	
+
+	if (seq->clip->id.us == 0)
+		seq->clip->id.us = 1;
+
 	/* basic defaults */
 	seq->strip = strip = MEM_callocN(sizeof(Strip), "strip");
 	seq->len =  BKE_movieclip_get_duration(clip);