Support for util ShaderCompilerSettings to Visitor and ConstVisitor

[theodoregoetz]

Visitor for ShaderCompilerSettings class as used in GraphicsPipelineConfigurator::traverse()

• New feature (non-breaking change which adds functionality)

This change allows a GraphicsPipelineConfigurator Visitor to apply to the shader hints:

struct SetPipelineStates : public vsg::Visitor
{
    void apply(vsg::Object& object) { object.traverse(*this); }
    void apply(vsg::ShaderCompileSettings& scs)
    {
        scs.defines.insert("VSG_TWO_SIDED_LIGHTING");
    }
} sps;
auto gpConf = vsg::GraphicsPipelineConfigurator::create(shaderSet);
gpConf->accept(sps);

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings