Fix or discard SLD Symbolizer Renderer

[slafayIGN]

The demo offers a switch between 2 symbolizer renderers : OpenLayers or SLD.

The OpenLayers renderer is generating an image client side and is working fine.

The SLD renderer makes use of a distant image generator (https://ows-demo.terrestris.de/geoserver/ows? with a GetLegendGraphic request) and displays broken images.

Finding and fixing the cause of this bug would be great.

Otherwise, the utility of keeping both symbolizer on the demo could be discussed.
Unless the OpenLayers symbolizer cannot display properly some specific styling feature, the SLD render could be discarded. It would remove a bit of complexity from the demo.