A more general `PLatLongEnvSphere`

[TzuChieh]

Move some of the geometry hacks in LatLongEnvEmitter as helper methods in PLatLongEnvSphere. PLatLongEnvSphere should stay general & low-level so more environment emitter can depend on it in the future.

Some helper methods of PLatLongEnvSphere can generate hit event.

This should be done before starting the update:

• [x] Add envieonment map render test.
• [x] Move direct PDF calculation to PLatLongEnvSphere.

[TzuChieh]

Additional issues found in the process:

• [x] PDF used is for infinite area light. We need to account the radius in Photon as our environment light do have a specific size (LatLongEnvEmitter::emitRay() and LatLongEnvEmitter::genDirectSample()).

[TzuChieh]

I always thought my environment light implementation is slightly off--the noise level just doesn't seem right (just a feeling). Now I think there is actually something wrong: the environment sphere is being modified to have a finite size in an update, and the PDF should not depend on its direction only. Where is the pdfA -> pdfW cosine term? Oh my god....

The default size of the environment sphere is quite large though (> 1km, most cosine terms should be ~1; nevertheless, this is an error).

[TzuChieh]

Should be corrected at this point 95805a3.