implemented visibility flags for object instances.

by default, all flags are set, i.e. we have

    <object_instance name="myInstance" object="myObject">
        <parameters name="visibility">
            <parameter name="camera" value="true" />
            <parameter name="light" value="true" />
            <parameter name="shadow" value="true" />
            <parameter name="transparency" value="true" />
            <parameter name="probe" value="true" />
            <parameter name="diffuse" value="true" />
            <parameter name="glossy" value="true" />
            <parameter name="specular" value="true" />
        </parameters>
    </object_instance>