Cleanup messy code during the release of v2.0.0-beta

[TzuChieh]

Some parts of the codebase are messed up to quickly support some features:

• renderer
• static canvas display in studio
• estimators

these should be cleaned up or refactored

[TzuChieh]

Specific list of things to do:

• [ ] Revisit the implementation of sampling renderer (especially the part for camera measurement)
• [x] Redesign the asynchronous render progress report mechanism
• [x] Separate the implementation of various photon mapping techniques