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().
Draft because it depends on a future libwayland release.
Edited by Jonas Ådahl