Refactor Vertex Color export and import

[sercero]

There were some issues with the code regarding Vertex Colors:

• The exporter was stuck with conventions of Blender 2.7x like alpha not being part of the Vertex Color data.
• In Blender 3.2+ the mesh now has color_attributes instead of vertex_colors
• The importer had a bug where an unknown variable was being referenced.

[sercero]

@paroj is there a way to represent vertex colors with the RTSS?

And also per vertex transparency?

[paroj]

the RTSS follows the normal diffuse colour flow, which can track vertex colours and contain alpha. See (vertexcolour):
https://ogrecave.github.io/ogre/api/latest/_material-_scripts.html#autotoc_md128