fix display of panels that use the bufferedcanvas (again)

Re-applying my commit b232bb89898da0acff2b97bd460b801cbace1019
It has been undone by c17bc11169522d6d38436f84fd08baa79e914480
As the later commit had the title 'Slic3r integrated for
Ubuntu 64bit', I don't think this was by intention. Therefore
I redo my change. (Without it pronterface on my PC does not
display the temperature graph, the xy- and z- buttons.)