limit the maximum of rasterized image to 4096 pixles.

XXX since the dimention can be different on each page, it is better to
call 'pdfinfo -f page_num -l page_num' to get the size of each page.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29404 20353a03-c40f-0410-a6d1-a30d3c3de9de