EGLFS: Fix plugin destruction

The proper init/destruction order is as follows:

QEglFsHooks::platformInit()
eglInitialize()
eglTerminate()
QEglFsHooks::platformDestroy()

Prior to this patch platformDestroy() was called before eglTerminate(),
leading to a crash on some platforms.

Additionally we need to destroy the native windows before deleting the
screen, otherwise the QEglFSWindow destructor ends up calling into the
deallocated screen.

Change-Id: Id08ccbac9bb44a778bcf1a55f73c0057e0a7b3af
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>