Metaball tessellation optimization (Octree to BVH)

Speedup is non-linear, 2x-10x faster is quite normal.
Patch T43678.

- Switched from an Octree to BVH.
- Finding first points of surface no longer "wastes" density function evaluation: every result is cached.
- Use MemArena instead of using own memory management.
- Correct calculation of metaelem bounding box.
- Remove mball_count(): mballs are now counted "on the go".

source/blender/blenkernel/BKE_mball.h

@@ -45,8 +45,6 @@ struct MetaBall *BKE_mball_copy(struct MetaBall *mb);
 
 void BKE_mball_make_local(struct MetaBall *mb);
 
-void BKE_mball_cubeTable_free(void);
-
 bool BKE_mball_is_basis_for(struct Object *ob1, struct Object *ob2);
 bool BKE_mball_is_basis(struct Object *ob);
 struct Object *BKE_mball_basis_find(struct Scene *scene, struct Object *ob);

source/blender/blenkernel/BKE_mball_tessellate.h

@@ -31,4 +31,6 @@ void BKE_mball_polygonize(
         struct EvaluationContext *eval_ctx, struct Scene *scene,
         struct Object *ob, struct ListBase *dispbase);
 
+void BKE_mball_cubeTable_free(void);
+
 #endif  /* __BKE_MBALL_TESSELLATE_H__ */

source/blender/windowmanager/intern/wm_init_exit.c

@@ -60,7 +60,7 @@
 #include "BKE_global.h"
 #include "BKE_library.h"
 #include "BKE_main.h"
-#include "BKE_mball.h"
+#include "BKE_mball_tessellate.h"
 #include "BKE_node.h"
 #include "BKE_report.h"