Make LineShape2D normal point upwards by default

Allows line shapes to collide with objects falling from the top by
default, which makes more sense for the most common cases.

doc/classes/LineShape2D.xml

@@ -14,8 +14,8 @@
 		<member name="distance" type="float" setter="set_distance" getter="get_distance" default="0.0">
 			The line's distance from the origin.
 		</member>
-		<member name="normal" type="Vector2" setter="set_normal" getter="get_normal" default="Vector2(0, 1)">
-			The line's normal.
+		<member name="normal" type="Vector2" setter="set_normal" getter="get_normal" default="Vector2(0, -1)">
+			The line's normal. Defaults to [code]Vector2.UP[/code].
 		</member>
 	</members>
 	<constants>

scene/resources/line_shape_2d.h

@@ -36,7 +36,8 @@
 class LineShape2D : public Shape2D {
 	GDCLASS(LineShape2D, Shape2D);
 
-	Vector2 normal = Vector2(0, 1);
+	// LineShape2D is often used for one-way platforms, where the normal pointing up makes sense.
+	Vector2 normal = Vector2(0, -1);
 	real_t distance = 0.0;
 
 	void _update_shape();