Allow transparent backgrounds on reports added to presentation slides

Some of the charts (e.g. Pies) have a ton of white space around them. When placing multiple charts on a single slide space is precious and we're forced with either making charts super small or having them to overlap each other. With 2 or 3 charts per slide it's sometimes possible to use back/foreground to get around this limitation. Harder to do with numerous chart on a single slide.