update debug shader

src/default_shaders.cpp

@@ -118,8 +118,9 @@ uniform mat3 normal_matrix;
 
 void main()
 {
-	color = vec4(0.5 * normalize(fragNormal) + 0.5, 1.0);
-	//color = vec4(1.0, 0.8, 0.8, 1.0);
+	color = vec4(normalize(
+			vec3(fragNormal.x, fragNormal.y, -1 * fragNormal.z)
+		), 1.0);
 }
 )DFS";