draw.h: add DefaultDPI

R=rsc
http://codereview.appspot.com/6858071
diff --git a/include/draw.h b/include/draw.h
index 3e21ff1..9bf5ce1 100644
--- a/include/draw.h
+++ b/include/draw.h
@@ -62,7 +62,8 @@
 {
 	Displaybufsize	= 8000,
 	ICOSSCALE	= 1024,
-	Borderwidth =	4
+	Borderwidth	= 4,
+	DefaultDPI	= 100
 };
 
 enum