eglfs: Implement nativeResourceForWindow

Add support to request EglDisplay resource for a window. This is
supported in other platform plugins,  like kms and xcb and is required by
wayland-egl based QtWayland compositors.

(cherry-picked from qtbase/fe375dc3d0ae28df6d929a5f86fd0a162a898853)
Return EGLNativeWindowType instead of window number in winId()

QEglFSWindow::winId() method was changed to return EGLNativeWindowType
EGL window handle instead of static window number as it recommends in
documentation.

QPlatformWindow documentation reads: "The platform
specific window handle can be retrieved by the winId function." and
also for winId() method itself: "Reimplement in subclasses to return
a handle to the native window".

(cherry-picked from qtbase/071c48a5ff8842d43fcdc95db08b561d340a2bfd)
Check for a valid window handle before attempting to retrieve resources.

In the case of a window that is not yet shown, or destroyed, there will not be a
native handle, and this would crash.
Merge pull request #10 from rburchell/new-qtwayland

Changes required for new QtWayland.