Render Task uses Sample by reference.
Support for random and stratified sampling
updated DOF test
lens sampling works again (was disabled as sampling was introduced)
added grid material
use information of previous intersection to prevent hitting the same area of surface twice
short circuit path when bsdf resolves to zero (e.g. when TIR occurs for transmission)
added non-reflective glass material (for testing transmission)
added Color skylight
Merge branch 'acne'