X11 support

[l4l]

This should be done in a conflicting feature (i.e. binary either build with wayland support or with x11 but not both).

I believe (hope) sdl2 might nicely fit the current implementation. All that should be done:

• main refactored a bit (i.e. wayland/x11 related things better be in other module);
• rendering should be directed to Surface::without_lock_mut;
• input events are mapped appropriately (i.e. sctk::seat::keyboard::Event to sdl2::event::Event).