UI: reduce area splitting action zone size.

This would overlap with buttons in the header. It's smaller to hit, but
still wider than the outlines for resizing, so hopefully it's fine.

Differential Revision: https://developer.blender.org/D4033

source/blender/editors/screen/screen_intern.h

@@ -37,7 +37,7 @@ struct Main;
 
 /* internal exports only */
 
-#define AZONESPOT       (0.8f * U.widget_unit)
+#define AZONESPOT       (0.4f * U.widget_unit)
 #define AZONEFADEIN     (5.0f * U.widget_unit) /* when azone is totally visible */
 #define AZONEFADEOUT    (6.5f * U.widget_unit) /* when we start seeing the azone */