Correct center for scroll zoom

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36559 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
sum[sum[x+2y, x, 1, 3], y, 2, 4] 

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36560 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Use @ rather than % in Sum variables to avoid syntax problems

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36561 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e