DropShadowFilter does not work with InstancedGeometry

[Ali-RS]

Hi,

For instanced geometries, no shadow is rendered under the objects.

in the below line

SimFX/src/main/java/com/simsilica/fx/shadow/DropShadowFilter.java

Lines 269 to 271 in f39387f

	for( int i = 0; i < size; i++ ) {
	Geometry g = casters.get(i);

perhaps we should check if the geometry is an InstancedGeometry and then check its instance list as well.

if (g instanceof InstancedGeometry ig) {
     for (Geometry instance : ig.getGeometries()) {
          // process instance 
     }
}