actually using the program we made is helpful

template/test.cc

@@ -175,6 +175,7 @@ bool init(int width, int height)
 void display(void)
 {
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+    glUseProgram(program);
     glBindBuffer(GL_ARRAY_BUFFER, data_vbo);
     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_vbo);
     glEnableVertexAttribArray(LOC_POSITION);