Fix export in JSurface for contour and density plots

[GoogleCodeExporter]

printHeight and printWidth needs to be set when calling export in JSurface 
since printing=true and computePlotArea has the following code:

        if (printing) {
            width = printwidth;
            height = printheight;
        } else {
            width = getBounds().width;
            height = getBounds().height;
        }

see changesets on my clone
https://code.google.com/r/oyarzun-surfaceplotter/source/list

Original issue reported on code.google.com by oyar...@gmail.com on 14 Nov 2014 at 4:18