The order of the elements is irrelevant and duplicate elements are unwanted. It
is therefore a perfect candidate for a set instead of a list. This prevents
filtering for duplicates manually be replacing some filer codes with (more
performant) builtin methods of QSet.
Using this renderer no symbol will be drawn for features, but labeling,
diagrams and other non-symbol parts will still be shown.
Selections can still be made on the layer in the canvas and selected
features will be rendered with a default symbol. Features being edited
will also be shown.
This is intended as a handy shortcut for layers which you only want
to show labels or diagrams for, and avoids the need to render
symbols with totally transparent fill/border to achieve this.
(fix#12131)
