Add Custom Loop Normals.

This is the core code for it, tools (datatransfer and modifier) will come in next commits).
RNA api is already there, though.

See the code for details, but basically, we define, for each 'smooth fan'
(which is a set of adjacent loops around a same vertex that are smooth, i.e. have a single same normal),
a 'loop normal space' (or lnor space), using auto-computed normal and relevant edges, and store
custom normal as two angular factors inside that space. This allows to have custom normals
'following' deformations of the geometry, and to only save two shorts per loop in new clnor CDLayer.

Normal manipulation (editing, mixing, interpolating, etc.) shall always happen with plain 3D vectors normals,
and be converted back into storage format at the end.

Clnor computation has also been threaded (at least for Mesh case, not for BMesh), since the process can
be rather heavy with high poly meshes.

Also, bumping subversion, and fix mess in 2.70 versioning code.

release/scripts/startup/bl_ui/properties_data_mesh.py

@@ -154,7 +154,7 @@ class DATA_PT_normals(MeshButtonsPanel, Panel):
         col = split.column()
         col.prop(mesh, "use_auto_smooth")
         sub = col.column()
-        sub.active = mesh.use_auto_smooth
+        sub.active = mesh.use_auto_smooth and not mesh.has_custom_normals
         sub.prop(mesh, "auto_smooth_angle", text="Angle")
 
         split.prop(mesh, "show_double_sided")
@@ -372,6 +372,11 @@ class DATA_PT_customdata(MeshButtonsPanel, Panel):
         col.operator("mesh.customdata_clear_mask", icon='X')
         col.operator("mesh.customdata_clear_skin", icon='X')
 
+        if me.has_custom_normals:
+            col.operator("mesh.customdata_custom_splitnormals_clear", icon='X')
+        else:
+            col.operator("mesh.customdata_custom_splitnormals_add", icon='ZOOMIN')
+
         col = layout.column()
 
         col.enabled = (obj.mode != 'EDIT')