cosmetics.
decreased the maximum number of AOVs from 32 to 16.
print the size of the renderer::ShadingResult structure.
removed redundant comment.
made renderer::ImageStack non-copyable.
added foundation::compute_normalization_factor() to compute the normalization factor for a given filter.
added a test case.
support reconstruction filters in progressive renderers; major refactorings.

notable refactorings:

  - introduced the notion of "pixel renderers" (with two concrete implementation: a uniform pixel renderer and an adaptive pixel renderer) to separate concerns that were previously intertwined in the generic tile renderer.
  - introduced a filtered frame buffer object (renderer::FilteredFrameBuffer).
  - reconstruction filter settings (filter type and radius) are now defined in the frame (renderer::Frame).
  - refactored the renderer::AccumulationFrameBuffer class (now almost an interface).