Added VSG_ALPHA_TEST to built-in ShaderSets to improve performance

[robertosfield]

I have added a VSG_ALPHA_TEST defined the Flat_Shaded, Phong and PBR ShaderSets to make the alpha based discard optionally compiled in. This is similar to GL_ALPHA_TEST provided in OpenGL's fixed function path.

• Changed to VulkanSceneGraph
• Changes to vsgExamples

To help test the performance impact of making the discard optional I have added the following to the vsgshadow example:

    if (arguments.read({"--alpha-test", "--at"}))
    {
        shaderHints->defines.insert("VSG_ALPHA_TEST");
    }

The fragment shader code that is controlled by this define is:

#ifdef VSG_ALPHA_TEST
    {
    if (material.alphaMask == 1.0f && diffuseColor.a < material.alphaMaskCutoff) discard;
        if (diffuseColor.a < material.alphaMaskCutoff)
#endif

I have tested my AMD5700G Linux desktop (integrated GPU) and Intel i7 + Geforce 2060 Windows 11 system and see around 5% improvement with the new default configuration (without VSG_ALPHA_TEST) for the tests I've done.

I expect scenes with lots of overdraw when early depth test & discard GPU is most effective will see the most gains for this change, though I haven't done exhaustive tests with it yet. The results I've seen already given me enough confidence that switching off the above discard code-path by default is helpful for most applications.