There are two ways of taking "order" into account when rendering:

1. You can control the feature rendering order which essentially does an
   "order by" of your layer data.
2. You can enable symbol levels which defines which styles are rendered
   in which order.

This commit addresses a problem when trying to bring (1) and (2)
together. When both are enabled, the rendering order (2) trumps the
order given by (1), essentially ignoring the order from (1).

With this commit rendering instead starts "from scratch" every time the
"order by" attributes have a new value.

This allows, for instance, to render complex roads networks correctly.
Ordering via (1) is used to draw roads going over bridges etc. in the
correct order while ordering via (2) is used to render road styles
correctly (thicker dark line, then thinner light line to get a roads
with casing).

The patch works by putting all features that need to be rendered not in
a single QHash (Symbol->Feature) but a QList of those hashes. Every time
at least one of the attributes used for the "order by" changes, a new
element is added to that list. The patch is rather simple, it just looks
a bit larger due to the extra indentation needed.

I thought about making this configurable, but I don't see any use case
where it makes sense to have (1) and (2) enabled where you don't want
this behaviour.

Fixes #42428
See #17276