gdk/wayland/surface: Expose gdk_wayland_surface_get_wl_egl_window()

gdk/wayland/surface: Expose gdk_wayland_surface_get_wl_egl_window()

Some GTK based applications such as Qemu UI create and manage
EGLSurfaces associated with the relevant GdkSurfaces. In order to create
an EGLSurface, there needs to be a way to pass the native window
object to eglCreateWindowSurface(). While running in an X environment,
the native window object can be obtained by calling
gdk_x11_surface_get_xid(). Likewise, the native window object can be
obtained by calling gdk_wayland_surface_get_wl_egl_window() while
running in a Wayland environment. Therefore, this API needs to be
exposed to apps.

Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>