Improvements to Light Layer usage

[dougal2]

There are a couple of things regarding light layers that IMO could be improved:

1) Allow assigning explicit layer names to <env_map> and <background> tags, rather than dumping always into the first layer.

2) Change the layer syntax to use layer names directly rather than requiring an index number and separate <layer_name> tags. At run-time Indigo should index the used names automatically if needed.

The reasons for these changes are mostly due to ending up with <env_map> and <background> light sources always in the first layer, and being unable to remove that first layer if those tags are not used.

I don't think this is particularly a consequence of how Blendigo operates, but I'd welcome advice from other exporter devs if you've sucessfully worked around this issue.