2.5

Nice goodie: Preview renders!

- Added new preview.blend, allowing super wide cinemascope previews
- Draws nicely blended inside widget type, rounded corners
- Preview now renders using all available cpus/cores.
- Uses - hopefully rock stable - method, which doesn't copy or 
  allocate anything for previews, but just uses render API calls.
- Multiple previews are possible! But, added provision in Jobs
  manager to only render one preview job at a time. If you start
  more preview jobs, they're suspended until it's their turn.

Bugfix: new buttons context code crashed when going full-window.

Tweaks are still needed for notifiers. I have to figure out still
how to retrieve SpaceButs button view types...

source/blender/editors/space_node/node_edit.c

@@ -183,7 +183,7 @@ void snode_composite_job(const bContext *C, ScrArea *sa)
 	WM_jobs_timer(steve, 0.1, NC_SCENE, NC_SCENE|ND_COMPO_RESULT);
 	WM_jobs_callbacks(steve, compo_startjob, compo_initjob, compo_updatejob);
 	
-	WM_jobs_start(steve);
+	WM_jobs_start(CTX_wm_manager(C), steve);
 	
 }