Collision: skip expensive BVH update if the collider doesn't move.

Since the collision modifier cannot be disabled, it causes a constant
hit on the viewport animation playback FPS. Most of this overhead can
be automatically removed in the case when the collider is static.

The updates are only skipped when the collider was stationary during
the preceding update as well, so the state is stored in a field.
Knowing that the collider is static can also be used to disable similar
BVH updates for substeps in the actual cloth simulation code.

Differential Revision: https://developer.blender.org/D2277

source/blender/blenkernel/intern/collision.c

@@ -71,6 +71,15 @@ void collision_move_object(CollisionModifierData *collmd, float step, float prev
 	float tv[3] = {0, 0, 0};
 	unsigned int i = 0;
 
+	/* the collider doesn't move this frame */
+	if (collmd->is_static) {
+		for (i = 0; i < collmd->mvert_num; i++) {
+			zero_v3(collmd->current_v[i].co);
+		}
+
+		return;
+	}
+
 	for (i = 0; i < collmd->mvert_num; i++) {
 		sub_v3_v3v3(tv, collmd->xnew[i].co, collmd->x[i].co);
 		VECADDS(collmd->current_x[i].co, collmd->x[i].co, tv, prevstep);