dot：最简单的光照

void main() {
  vec3 normal = normalize(vNormal);
  vec3 light = normalize(vec3(-0.600, 0.800, 1.0));
  float diffuse = max(dot(normal, light), 0.0);
  vec3 color = vec3(0.96, 0.43, 0.30) * (0.120 + diffuse * (1.0 - 0.120));
  gl_FragColor = vec4(color, 1.0);
}