implement DirectFB::Surface#lock and DirectFB::Surface#unlock
update example/surface.rb
implement DirectFB::Surface#get_framebuffer_offset
fix include/directfb_event_buffer.h
implement mrb_directfb_window_description_get()
implement DirectFB::Window
def DirectFB::Window
implement DirectFB::DislayLayer#create_window and DirectFB::DisplayLayer#get_window