Pinch zooming to the pointer on an infinite canvas. We're having to offset the camera as we change the zoom, and that offset amount was... not intuitive.
With this kind of thing, it's useful to think in terms of separate "screen space" and "document space" coordinates. These are usually very different: a pointer at x=100 y=100 on the screen could be anywhere in the document, depending on how the document is scrolled or zoomed.
The goal here is to *preserve* that document coordinate as the user zooms in or out. So as we zoom, we need to scoot the document over so that the user's pointer is still pointing at the same place in the document.
Here's a maybe cleaner look at what we're doing. To solve for the camera's next offsets, we need to know the camera's point, the pointer's point, the previous zoom and the difference (or delta) between the next zoom and the previous zoom. 
(If we weren't clamping the zoom, then we could just use the incoming `change` parameter. But since we are clamping, the actual delta might be reduced if we've had to clamp the result)
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