DumpRenderTRee: avoid crash at exit.

The default behavior of QuitRequested, closing and deleting all windows,
is the right thing to do. If we leave windows opened, they are
destroyed later in the exit phase and this leads to a crash as they
access things from WebKit that are not available anymore.
ImageBuffer: better handling of allocation failure

Report the error to the caller, rather than ASSERTing. However, not all
callers seem to handle this properly.