Ambient-occlusion map not exported

[sercero]

Support for Metal-roughness maps was added based on the following topic: #138

Now, there is Ambient-occlusion missing which can be implemented by using a separate grayscale texture with modulate.

Some comments from: https://matrix.to/#/!DzKeCPZzhuYhzPSQfP:gitter.im/$J5f-KtsWIx5k53_FXaWuh2QWSg2iRlElMfFcl4t3m-o?via=gitter.im&via=matrix.org&via=the-gdn.net

storing it in the metalRoughness image, is currently not supported: https://github.com/KhronosGroup/glTF/issues/1593#issuecomment-479999422 however, just using a separate grayscale texture with modulate should work

you dont need the RTSS for baked AO - just put it in a normal texture_unit {..} section