mem leak in snapping.

Steps to reproduce:

1. open the provided file.
2. turn on snapping
3. drag rectangle around the top left corner of the page border

What happened?

Inkscape hangs when snapping is turned on this PDF import, consuming ~790MB/s of memory until gets killed by oom-killer.

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.2-dev (963d16bd, 2021-07-10, custom)
• Operating System: ArchLinux
• Operating System version: build dependencies of inkscape-git package
• Offending file (just a fragment that can induce the issue) _test.svg
• Heaptrack graph