Fix: 3D Viewport header missing update changing paintslots

Since 3b5df8a7ea, we are displaying a "canvas picker" in the 3DView
header. In texture paint mode, this gives us the active paintslot to
paint on, however the header wasnt updating to another slot.

See https://projects.blender.org/blender/blender/pulls/
112227#issuecomment-1018440

Part of #112186.

Pull Request: https://projects.blender.org/blender/blender/pulls/115802

source/blender/editors/space_view3d/space_view3d.cc

@@ -1610,6 +1610,12 @@ static void view3d_header_region_listener(const wmRegionListenerParams *params)
         ED_region_tag_redraw(region);
       }
       break;
+    case NC_MATERIAL:
+      /* For the canvas picker. */
+      if (wmn->data == ND_SHADING_LINKS) {
+        ED_region_tag_redraw(region);
+      }
+      break;
   }
 
     /* From top-bar, which ones are needed? split per header? */