implemented fractional sample counts for direct lighting and IBL in the distributed ray tracing and path tracing engines.

other changes:

  - the direct lighting integrator no longer switches to the one-sample model when the number of BSDF samples and the number of light samples are both 0; the one-sample model can now invoked using the renderer::DirectLightingIntegrator::take_single_bsdf_or_light_sample() method (but it is no longer available from the DRT and PT engines).
updated copyright notice.
added missing #include statement.
updated sun light model after recent changes to the physical sky models in 17c9618102670bf8eeccf874688424d64f3e4b50.
updated physical sun & sky test scenes.