Set canvas size for canvases with choices

[donaldgray]

Follow on from #204. This is to handle the possibility for items with a choice_order being slightly different sizes.

If the canvases returned as part of the NQ have slightly different sizes, set the width of the containing canvas to the largest overall width and the height to the largest overall height (even if these are from different images).

Note

This is only for canvasPaintings that don't have a target! We will handle those with a target later