wayland-egl: rework and simplify wl_egl_window initialization

wayland-egl: rework and simplify wl_egl_window initialization

Use calloc instead of malloc + explicitly zeroing the different fields.
We need special handling for the version field which is of type
const intptr_t.

As we're here document why keeping the constness is a good idea.

The wl_egl_window_resize() call is replaced with an explicit set of the
width/height.

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