… collection results

Fixes #35356

But in general, this is exciting because it allows QgsSymbol to correctly render any
input geometry types, and handle collections nicely. In future releases we could quite
easily add a symbol layer type for rendering collections, where the symbol layer has
a marker/line/fill symbol set, and the appropriate selection of these is used to
render each geometry part (it's basically identical to a geometry generator!)

Geometry collection handling at a QgsVectorLayer level, now that's a totally
different beast....