zooming more than one level per game update has weird behavior

[tlongstretch]

I noticed this scrolling the mousewheel to zoom. If you scroll aggressively it results in jumpy zoom and zooms the opposite way sometimes. I think this is caused by more than one zoom change per game update. As a work-around I tried limiting it to one zoom per game update, which fixes the jumpiness, though it would be nice to be able to zoom quickly. There may be other issues with more than one SDL event per game update