UI - LOCAL View3D overlay stats

This patch improves the 3DView statistics overlay to show LOCAL stats
while in local view. This means the stats can vary between 3DViews and
the statusbar when views are in local view, but this gives a much more
accurate count of the objects, and their components, that you are
directly working with rather than just scene values.

Differential Revision: https://developer.blender.org/D8883

Reviewed by Campbell Barton

source/blender/makesdna/DNA_view3d_types.h

@@ -258,6 +258,8 @@ typedef struct View3D_Runtime {
   int flag;
 
   char _pad1[4];
+  /* Only used for overlay stats while in localview. */
+  struct SceneStats *local_stats;
 } View3D_Runtime;
 
 /** 3D ViewPort Struct. */