Now have multiple lights.

13-multiple-lights/tests/bad_shader.vs

@@ -0,0 +1,10 @@
+#version 330 core
+layout (location = 0) in vec3 aPos; // the position variable has attribute position 0
+
+out vec4 vertexColor; // specify a color output to the fragment shader
+
+void main()
+// syntax error here
+    gl_Position = vec4(aPos, 1.0); // see how we directly give a vec3 to vec4's constructor
+    vertexColor = vec4(0.5, 0.0, 0.0, 1.0); // set the output variable to a dark-red color
+}