Max preview size to 4096x4096

With HiDPI screens. And even normal HD screens you want more detail from
your pictures. Or the ability to somewhat zoom on you previews. For this
we need somewhat larger previews.

Moving the default to 4096x4096 is a step up. Users that want the old
behavior can still set the values in config.php

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>