Get textures working in scene_synthesizer example

[RussTedrake]

I've opened https://github.com/NVlabs/scene_synthesizer/issues/10 to ask about it, but currently understand that the textures only work when exporting to USD.

A possible next step is to see how close Drake's fledgling USD parser is to parsing the generated scenes. (I tried a quick experiment on that, but even after building Drake using WITH_USD=on, calling the USD parser from installed python immediately throws an error about not being able to find runfiles.