Implement Stratified Sampling of Projected Spherical Caps

[oktomus]

See http://www.iliyan.com/publications/ProjectedSphericalCaps See https://github.com/est77/appleseed/tree/light_shapes_PR

• [x] Introduce procedural shapes -> #2521
• [x] Rename EmittingTriangle to EmittingShapes -> #2533
• [x] Use uniform sampling on triangles -> #2554
• [x] Use uniform sampling on rectangles ->#2576
• [x] Use uniform sampling on disks and spheres ->#2583
• [x] Make use of shape_prob and m_shape_prob coherent -> #2606
• [ ] Try to estimate the emission of each shape and use that in the CDF
• [ ] Use solid angle sampling on triangles
• [ ] Use solid angle sampling on disks
• [ ] Use solid angle sampling on spheres -> @oktomus is on it
• [ ] Use solid angle sampling on rectangles
• [ ] Use stratified sampling of PSC on spheres -> @oktomus is on it
• [ ] Add support for moving lights. See todo: add support for moving light-emitters.

[oktomus]

For solid angle sampling pdf issue, see:

• PathVertex::get_bsdf_prob_area

The thing is that some probs are on the area and some are on the solid angle.