Rework of BKE's mesh_render to support BMesh directly.

Note that since there is no (efficient) ways to get arrays of
MVert/MEdge/etc. out of a BMesh, I refactored quite heavily internals of
BKE_mesh_render.

Now, when you do need acess to mesh data to generate cached batches, you
create an abstract struct from mesh (either Mesh or BMesh if available),
and then use advanced helpers to extract needed data, on a per-item
basis (no more handling of arrays of verts/edges/... in batches code).

This allows to:
* Avoid having to create arrays of BMesh elements.
* Take advantage of existing advanced BMesh topology and connectivity data.

Reviewers: dfelinto, fclem, merwin

Differential Revision: https://developer.blender.org/D2521

source/blender/blenkernel/BKE_mesh_render.h

@@ -33,6 +33,7 @@ struct Batch;
 struct Mesh;
 
 void BKE_mesh_batch_cache_dirty(struct Mesh *me);
+void BKE_mesh_batch_cache_clear(struct Mesh *me);
 void BKE_mesh_batch_cache_free(struct Mesh *me);
 struct Batch *BKE_mesh_batch_cache_get_all_edges(struct Mesh *me);
 struct Batch *BKE_mesh_batch_cache_get_all_triangles(struct Mesh *me);