Fix crash when loading more than 16 vector layers

[uclaros]

Description

This fixes another unreported regression introduced by #50381. QGIS would crash when trying to load more than 16 vector layers at once.

This was caused by an ambiguous sorting rule passed to std::sort() for sorting vector layers based on their geometry type, which would cause the sorting algorithm to try to read past the list's last element.
Sorting 16 or less layers would not crash as std::sort() uses a different sorting algorithm for small lists.

Also, fixes proper ordering for multipolygons/multisurface/curvepolygons:
Before:

After:

Kudos to @wonder-sk for tracking this down!