compositor: Add support for wl_surface.offset()

This allows for setting a buffer offset without having to make it part
of the wl_surface.attach request. This is useful for e.g. setting a DND
surface icon hotspot offset when using Vulkan; or doing the same with
EGL without having to use wl_egl_window_resize().

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>