[skip travis] why doesn't appveyor restart?

taichi/backends/opengl/opengl_api.cpp

@@ -271,6 +271,7 @@ bool initialize_opengl(bool error_tolerance) {
     int status = glfwGetError(&desc);
     if (error_tolerance && status == GLFW_API_UNAVAILABLE) {
       // error tolerated, returning false
+      // note that we only tolerate GLFW_API_UNAVAILABLE
       TI_TRACE("GLFW: OpenGL API unavailable");
       supported = std::make_optional<bool>(false);
       return false;