Mesh shader support

[Robert Osfield]

Hi All,

I merged NVidia Mesh Shader extension with VulkanSceneGraph master,  The PR with this work is:

     https://github.com/vsg-dev/VulkanSceneGraph/pull/267

NVidia introduction to Mesh Shaders:

    https://developer.nvidia.com/blog/introduction-turing-mesh-shaders/

Demo of Mesh Shaders:

    https://www.youtube.com/watch?v=CRfZYJ_sk5E&ab_channel=NvidiaGameWorks

A much more humble demo/example is the vsgmeshshader example I've checked into vsgExamples master:

    https://github.com/vsg-dev/vsgExamples/blob/master/examples/rtx/vsgmeshshader/vsgmeshshader.cpp

All you'll get to see is a simple RGB triangle, but it does illustrate all the parts necessary to request mesh shader support from Vulkan instance/device. There is no  geometry  in the scene graph, the triangle generated is created entirely in the mesh shaders, vertex data and primitives all created procedurally in the shader - with no need for vertex, tessellation or geometry shader.

Right now it looks like only modern NVIdia GPUs support mesh shaders, all RTX consumer and professional cards support so 2060 onwards/upwards.  My mid-range Geforce 1650 also support mesh shaders, even though they don't support the ray tracing support found in RTX class cards.

As part of this work I've moved the RTX ray tracing classes into include/vsg/rtx to sit alongside the new vsg::DrawMeshTasks* classes.