Make the size of clipping masks only use the size of the mask

[WestonThayer]

The size of a clipping mask according to Illustrator's API is determined by the largest object in the clip group. This is super unintuitive, and they don't provide a nice way to get the size of the mask object.

• We can't simply ask for the first group child and take it's size, the mask object can be at any Z
• Clip groups can be nested, so we would have to recurse through the entire tree to get the true size