Default to $retinafy = true()

[greystate]

It's far more common to use retinafied images than not, so let's flip this one.

It's technically a breaking-change, but probably wont hurt any sites, should they decide to upgrade - plus, it's very easy to get the previous behavior, by adding <xsl:with-param name="retinafy" select="false()" /> to any call to the helper.