USD -> SDF converter: Find a better way to detect root model and light parent prims

[adlarkin]

Description

When converting lights attached to a USD stage to SDF, we assume that the parent of the USD light is within the first two layers of the USD stage (the light's parent is needed so we can compute the proper transform). This may not work for lights attached to models if the model is defined deeper in the USD stage.

One possible solution would be to traverse up the prim tree whenever a light prim is encountered. The traversal would stop whenever we come across a model prim or something like a ground plane/the root of the stage itself.

See gazebosim/sdformat#875 (comment) for more information.