Store aspect_ratio instead of weird proportional_{width,height} (Thanks phillip)

Store aspect_ratio instead of weird proportional_{width,height} (Thanks phillip)

This commit only goes to “next” because I am not sure whether it
actually makes things better in all cases and want to give it some
testing first.

There was no documented reason behind using the
proportional_{width,height} variables, so I suppose that code was just
stupidity on my part (it was written merely a month after I started this
project in 2009).

fixes #1032