Implement importance sampling for NoHit

[nonl4331]

Description
Currently, only Primitives are able to be importance sampled. Also being able to sample NoHit which often is a source of lighting would help scenes converge faster.

Tasks

• Discrete 1D and 2D PDF sampling
• Tests for 1D and 2D Sampling
• Generate 2D (UV) PDF from Texture
• Implementation for Sky
• Tests for NoHit sampling (for Sky)
• MIS code

[nonl4331]

• Investigate sky_sampling test converges incorrectly with large amount of samples. #2
  This is low priority and should not block merging into the main branch.