wayland-egl: Make wl_egl_window a versioned struct

wayland-egl: Make wl_egl_window a versioned struct

We need wl_egl_window to be a versioned struct in order to keep track of
ABI changes.

This change makes the first member of wl_egl_window the version number.

An heuristic in the wayland driver is added so that we don't break
backwards compatibility:

 - If the first field (version) is an actual pointer, it is an old
   implementation of wl_egl_window, and version points to the wl_surface
   proxy.

 - Else, the first field is the version number, and we have
   wl_egl_window::surface pointing to the wl_surface proxy.

Signed-off-by: Miguel A. Vico <mvicomoya@nvidia.com>
Reviewed-by: James Jones <jajones@nvidia.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>