Show warning when printing or pdfing a composition when blend modes are present

Since QPainter doesn't support composition modes, this adds a warning message to tell the user to "print as raster" when a composition contains blending and they are attempting to print or pdf it.

(Discussed on qgis-dev here
http://lists.osgeo.org/pipermail/qgis-developer/2013-May/026001.html)