[labeling] Render Vector Tile labels on the same z-level as their layer

[YoannQDQ]

Feature description

Vector Tile Layers can be used as basemaps that always look crisp, regardless of map rotation, crs. Problem is that the vector tile labels (as all labels) appear above other layers.

Here's an example, with a base map vector tile layer (osm, Positron style), and a bogus data layer (concentric blue circles)

It would be great if labels on vector tile layers could be rendered on the same z-level as their layer.

Expected result

This can be achieved in the print layout by using two overlaping maps with different themes (one with the vectortile layer, and the other with the other layers), but it is really tedious, as the two maps must be kept in sync.

This FR would also be adressed by #25574, but is more restricted in scope.

Additional context

No response