Compositing updates

Add DPI support for composited image, also fixed a bug in the image resizing code. It was using the report lab inch (which is 74.0) to work out the image scale factor and meant images were being resized to too small - a 300 dpi image was 260-somthing-px wide, when it should be 750px.

Let me know if I'm off base with these / have broken things.