more sppm work.
fixed evalution of BSDF inputs in SPPM lighting engine.
more sppm work.
renderer::PixelRendererBase now provides empty implementations of on_tile_begin() and on_tile_end().
renderer::IPassCallback::pre_render() now takes the frame in argument.
removed empty file.
formatting.
added missing #include statements.
renderer::IPassCallback::pre_render() now takes a *constant* reference to the frame.
switched from QMC sampler to RNG sampler.