Use triangle strip stead of quad

main.c

@@ -218,7 +218,7 @@ int main()
         glClear(GL_COLOR_BUFFER_BIT);
 
         if (!program_failed) {
-            glDrawArrays(GL_QUADS, 0, 4);
+            glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
         }
 
         glfwSwapBuffers(window);

main.vert

@@ -2,11 +2,9 @@
 
 void main(void)
 {
-    int gray = gl_VertexID ^ (gl_VertexID >> 1);
-
     gl_Position = vec4(
-        2 * (gray / 2) - 1,
-        2 * (gray % 2) - 1,
+        2 * (gl_VertexID / 2) - 1,
+        2 * (gl_VertexID % 2) - 1,
         0.0,
         1.0);
 };