EGL: Add support for ws_DestroyWindow

This is mostly needed so we can call ```eglCreateWindowSurface``` and ```eglDestroySurface``` in alternating fashion without all calls but the first failing (with ```EGLPLATFORM=fbdev```).

However, it might also make sense for other platforms to clean up resources allocated in ```CreateWindow``` (see ```TODO``` comments in the platforms for which I wasn't sure what kind of clean-up is necessary).