Improve the architecture and implementations of emitters

[TzuChieh]

• cleaner cooking process
• better handling rotation and different uv mapping for background emitter
• able to rotate IES light modulators

[TzuChieh]

• How do we test for sidedness agreement on emitters?

• [ ] Emitter implementations should test for sidedness on their own. They should be the one answering "Can I emit in this direction?", "Can I sample this point?".

[TzuChieh]

• [x] Using new query scheme for DirectEnergySampleQuery
• [x] Add DirectEnergySamplePdfQuery
• [ ] EmitterSampler should be in charge of sampling from group of emitters. Scene provides helper methods only.
• [x] Add render test for environment map