Collision Shape 2D 'Disabled' Visualization Correction

Having white or strongly desaturated debug collision shape color
setting would make it harder to visualize enabled / disabled state.
This change makes it easier to visualize enabled / disabled state
by reducing the alpha color by half when disabled.

(cherry picked from commit 0c4594f6c9)

scene/2d/collision_shape_2d.cpp

@@ -117,6 +117,7 @@ void CollisionShape2D::_notification(int p_what) {
 				draw_col.r = g;
 				draw_col.g = g;
 				draw_col.b = g;
+				draw_col.a *= 0.5;
 			}
 			shape->draw(get_canvas_item(), draw_col);