Constant emissive colors

[MeineZ]

The feature
WispRenderer supports emissive textures and emission weight. However, as for every other material texture attribute, there's also a constant variant. The feature request would be to have support for a constant emissive color as well. A big bonus would be that the weight could also be set as a texture.

Additional context
This feature request comes from WispForMaya. Maya supports this request, but WispForMaya can't support it yet as WispRenderer doesn't support it.