Support MarkersSymbolizer in GroupSymbolizer

The motivation of these changes is to support markers symbolizer inside of group symbolizer, so that point symbolizer can be removed without breaking group symbolizer tests. This is a requirement for #952.

Some refactoring of marker handling was needed to avoid a lot of code duplication for marker rendering in group symbolizer. The refactor also helped to eliminate some existing code duplication between renderers for the marker dispatch types.

I also added a new test case to test different types of markers (ellipse, vector, raster, etc) within group symbolizer.