Thread rendering stability commit.

I noticed still several cases where the Imbuf library was called within a
thread... and that whilst the Imbuf itself isn't threadsafe. Also the
thread lock I added in rendering for loading images actually didn't
work, because then it was still possible both threads were accessing the
MEM_malloc function at same time.

This commit nearly fully replaces ImBuf calls in compositor (giving another
nice speedup btw, the way preview images in Nodes were calculated used
clumsy imbuf scaling code).

I've also centralized the 'mutex' locking for threading, which now only
resides in BLI_threads.h. This is used to secure the last ImBuf calls
I cannot replace, which is loading images and creating mipmaps.

Really hope we get something more stable now!

source/blender/blenlib/BLI_threads.h

@@ -38,7 +38,10 @@ int		BLI_available_thread_index(ListBase *threadbase);
 void	BLI_insert_thread	(ListBase *threadbase, void *callerdata);
 void	BLI_remove_thread	(ListBase *threadbase, void *callerdata);
 void	BLI_end_threads		(ListBase *threadbase);
-		
+
+void	BLI_lock_thread		(void);
+void	BLI_unlock_thread	(void);
+
 		/* threadsafe version of MEM_malloc and friends */
 void	*MEM_mallocT(int len, char *name);
 void	*MEM_callocT(int len, char *name);