Mesh Batch Cache: Refactor + Multithread

For clarity sake, the batch cache now uses exclusively per Loop attributes.
While this is a bit of a waste of VRAM (for the few case where per vert
attribs are enough) it reduces the complexity and amount of overall VBO
to update in general situations.

This patch also makes the VertexBuffers filling multithreaded. This make
the update of dense meshes a bit faster. The main bottleneck is the
IndexBuffers update which cannot be multithreaded efficiently (have to
increment a counter and/or do a final sorting pass).

We introduce the concept of "extract" functions/step.
All extract functions are executed in one thread each and if possible,
using multiple thread for looping over all elements.

Reviewed By: brecht

Differential Revision: http://developer.blender.org/D5424

source/blender/draw/intern/draw_cache_impl.h

@@ -119,7 +119,7 @@ struct GPUBatch *DRW_lattice_batch_cache_get_edit_verts(struct Lattice *lt);
 /* Mesh */
 void DRW_mesh_batch_cache_create_requested(struct Object *ob,
                                            struct Mesh *me,
-                                           const struct ToolSettings *ts,
+                                           const struct Scene *scene,
                                            const bool is_paint_mode,
                                            const bool use_hide);
 
@@ -143,6 +143,7 @@ struct GPUBatch *DRW_mesh_batch_cache_get_surface_weights(struct Mesh *me);
 struct GPUBatch *DRW_mesh_batch_cache_get_edit_triangles(struct Mesh *me);
 struct GPUBatch *DRW_mesh_batch_cache_get_edit_vertices(struct Mesh *me);
 struct GPUBatch *DRW_mesh_batch_cache_get_edit_edges(struct Mesh *me);
+struct GPUBatch *DRW_mesh_batch_cache_get_edit_vnors(struct Mesh *me);
 struct GPUBatch *DRW_mesh_batch_cache_get_edit_lnors(struct Mesh *me);
 struct GPUBatch *DRW_mesh_batch_cache_get_edit_facedots(struct Mesh *me);
 /* edit-mesh selection */