[FIXED] VSG_GREYSCALE_DIFFUSE_MAP typo

[siystar]

Fixed typo in VSG_GREYSCALE_DIFFUSE_MAP ShaderSet define. Matching changes would be needed in vsg/utils/Builder.cpp and shader blobs included by vsg/utils/ShaderSet.cpp (how to re-generate them?).

[siystar]

@robertosfield We will need to update code in vsg-dev/VulkanSceneGraph to match this PR's changes. What is the recommended way to re-generate the binary ShaderSet files in the VSG source tree?

[robertosfield]

I have a script that runs vsgshaderset for the various shader set combinations with the setting required to rebuild the most common variants. I don't have this script on my laptop so will post this once I'm back on my main desktop system. Longer term it might be something to put in CMake.

[robertosfield]

As a first step I've encoded the shell script I use to rebuild the built-in ShaderSets:
vsg-dev/VulkanSceneGraph@f903b1e

I ran this to update built-in ShaderSets which is reflected in the commit:
vsg-dev/VulkanSceneGraph@f903b1e

I think a good refinement of this would be to put support into vsgshaderset for loading all files from a directory and it's sub-directories so we can remove the find and xargs usage. Making the path to the directory glTF-Sample-Models, and control the other options passed to vsgshaderset would also make sense.