CanvasItemGroup::update is too slow to handle typical multi-page documents

Summary:

As of recent master ae62f47f, zooming/rotating a multi-page document is extremely slow, even with the changes of !3838 (merged). It turns out CanvasItemGroup::update is taking excessively long to update the geometry. This shows up as huge bars for on_idle, where this function is called:

Steps to reproduce:

• Open Inkscape.
• Open any PDF document containing more than one page in 'Internal import' mode.
• Attempt to zoom/rotate.

What happened?

Extremely low framerate, about 6 fps.

What should have happened?

A normal framerate, currently about 30-60 fps.

Other notes

• Splitting a multipage document into its constituent pages and viewing those doesn't result in any slowdown, suggesting the inefficiency lies purely within the multipage implementation.

• Huge kudos to @doctormo for this feature I've wanted for years!