[FIXED] #11572 Filter Legend By Map Content doesn't maintain point displacement legend

The fix from https://github.com/qgis/QGIS/pull/1673 has to be enhance to have
 something more generic.

This fix add methods to QgsPointDisplacementRenderer:
* capabilities
* symbolForFeature
* originalSymbolForFeature
* symbolsForFeature
* originalSymbolsForFeature
* willRenderFeature