Sample code for my 'set user data' issue

Minimal sample code to demonstrate the issue to use wl_output_set_user_data() and wl_surface_set_user_data().

Test environment:

• Fedora 25
• Gnome 3.22.2
• wayland-devel-1.12.0-1.fc25.x86_64
• libwayland-client-1.12.0-1.fc25.x86_64

Checkout, build and run:

$ git clone https://github.com/zhanggyb/set-user-data
$ mkdir build
$ cd build/
$ cmake ..
$ ./demo

When run the executable demo, assert error will be raised, and the const member variables around wayland object will be changed in the wl_output and wl_surface listener callbacks.

output.cpp:27: static void Output::OnGeometry(void*, wl_output*, int32_t, int32_t, int32_t, int32_t, int32_t, const char*, const char*, int32_t): Assertion `_this->placeholder1 == 0' failed.

surface.cpp:18: static void Surface::OnEnter(void*, wl_surface*, wl_output*): Assertion `_this->placeholder1 == 0' failed.

Uncomment these 2 lines can avoid assert error:

main.cpp:163: output.SetUserData(this);
main.cpp:164: surface.SetUserData(this);