Threaded object update and EvaluationContext

Summary:
Made objects update happening from multiple threads. It is a task-based
scheduling system which uses current dependency graph for spawning new
tasks. This means threading happens on object level, but the system is
flexible enough for higher granularity.

Technical details:

- Uses task scheduler which was recently committed to trunk
  (that one which Brecht ported from Cycles).

- Added two utility functions to dependency graph:
  * DAG_threaded_update_begin, which is called to  initialize threaded
    objects update. It will also schedule root DAG node to the queue,
    hence starting evaluation process.

    Initialization will calculate how much parents are to be evaluation
    before current DAG node can be scheduled. This value is used by task
    threads for faster detecting which nodes might be scheduled.

  * DAG_threaded_update_handle_node_updated which is  called from task
    thread function when node was fully handled.

	This function decreases num_pending_parents of node children and
	schedules children with zero valency.

    As it might have become clear, task thread receives DAG nodes and
    decides which callback to call for it.

    Currently only BKE_object_handle_update is called for object nodes.

    In the future it'll call node->callback() from Ali's new DAG.

- This required adding some workarounds to the render pipeline.
  Mainly to stop using get_object_dm() from modifiers' apply callback.
  Such a call was only a workaround for dependency graph glitch when
  rendering scene with, say, boolean modifiers before displaying
  this scene.

  Such change moves workaround from one place to another, so overall
  hackentropy remains the same.

- Added paradigm of EvaluaitonContext. Currently it's more like just a
  more reliable replacement for G.is_rendering which fails in some
  circumstances.

  Future idea of this context is to also store all the local data needed
  for objects evaluation such as local time, Copy-on-Write data and so.

  There're two types of EvaluationContext:

  * Context used for viewport updated and owned by Main. In the future
    this context might be easily moved to Window or Screen to allo
    per-window/per-screen local time.

  * Context used by render engines to evaluate objects for render purposes.
    Render engine is an owner of this context.

  This context is passed to all object update routines.

Reviewers: brecht, campbellbarton

Reviewed By: brecht

CC: lukastoenne

Differential Revision: https://developer.blender.org/D94

source/creator/creator.c

@@ -1258,6 +1258,7 @@ static int load_file(int UNUSED(argc), const char **argv, void *data)
 		 * pointcache works */
 		if (retval != BKE_READ_FILE_FAIL) {
 			wmWindowManager *wm = CTX_wm_manager(C);
+			Main *bmain = CTX_data_main(C);
 
 			/* special case, 2.4x files */
 			if (wm == NULL && CTX_data_main(C)->wm.first == NULL) {
@@ -1273,8 +1274,8 @@ static int load_file(int UNUSED(argc), const char **argv, void *data)
 			G.relbase_valid = 1;
 			if (CTX_wm_manager(C) == NULL) CTX_wm_manager_set(C, wm);  /* reset wm */
 
-			DAG_on_visible_update(CTX_data_main(C), TRUE);
-			BKE_scene_update_tagged(CTX_data_main(C), CTX_data_scene(C));
+			DAG_on_visible_update(bmain, TRUE);
+			BKE_scene_update_tagged(bmain->eval_ctx, bmain, CTX_data_scene(C));
 		}
 		else {
 			/* failed to load file, stop processing arguments */
@@ -1582,6 +1583,7 @@ int main(int argc, const char **argv)
 	IMB_init();
 	BKE_images_init();
 	BKE_modifier_init();
+	DAG_init();
 
 	BKE_brush_system_init();