mojave opengl fix

Former-commit-id: c271bbf50b18844b980d636a414fb19ed785f37b

include/igl/opengl/glfw/Viewer.cpp

@@ -145,6 +145,7 @@ namespace glfw
     #ifdef __APPLE__
       glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
       glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
+      glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, GLFW_TRUE);
     #endif
     if(fullscreen)
     {